Which monitor for Apple MacBook Pro?

@wmorg  You don't have ANY preferences or requirements around size, aspect ratio, resolution, refresh rate, etc.?  In terms of your requirements, the MacBook Pro 15" is designed for an 87W power source.  It will work with a lower wattage source, but you may see slower battery charging and/or reduced performance in that case.  If that isn't desirable, you'd have to look at displays that supply 90W or more.  There aren't very many of those within Dell's product line right now.  Most current displays only supply 65W.  But finding a display that has a USB-C input, built-in USB ports, and an HDMI input isn't all that difficult.  If you want 4K resolution, then if your 13" MacBook Pro has an 8th Gen CPU, be aware that you'd only have USB 2.0 data speeds available when connected over USB-C, due to bandwidth requirements of 4K and the DisplayPort revision that the 13" 2019 MBP w/ 8th Gen CPU supports.  (You'd also need a display that supported being configured to set up the USB-C link to maximize video bandwidth by only running USB 2.0 data.) If you use 1440p or below, you should be fine keeping USB 3.x.  Or if you have a 2019 MBP with a 10th Gen CPU, you'd be fine running USB 3.x even with 4K 60 Hz -- as long as you choose a display that supports DisplayPort 1.4/HBR3 on its USB-C input.

In terms of picture-by-picture, I pretty much only see that on ultrawide displays right now, like the U4919DW that has a resolution of 5120x1440 (it's essentially two regular 1440p panels fused together).  Picture-by-picture on a standard 16:9 display results in a pretty narrow workspace for each source.

Hopefully this gives you a few things to think about, but honestly you can find these specs just looking at the product pages of various displays.  And if you can't, go to support.dell.com, search the model of the display, go to the Documentation section, and open up the User Guide.  It will have more extensive detail.

Let me revise what I am looking for:

What I have:

1.  MacBookPro 13" 2019 

Graphics is Intel Iris Plus Graphics 645 1536 MB, Processor: 1.4 GHz Quad-Core Intel Core i5. It uses a 61W Adaptor

That is the one I want to have charged through the Monitor.

2,  MacBookPro 15" 2018 I think

I can charge that one through a Wall Outlet.

Size of the monitor: 27" or higher (not higher than 34". e.g 43" is too big.

Resolution around 1920 or higher.

I am ok with 4k, but not a strict requirement.

@wmorg  Ok, the Iris Plus 645 appears to be related to the Core 8th Gen CPU.  Would have been helpful to just provide more details on the CPU though, since that was the differentiating factor I mentioned.  In any case, if you want at least a 27" display, I would strongly encourage you to go for 2560x1440 resolution, not 1920x1080.  Having just 1920x1080 resolution spread across a 27" display results in a very low pixel density of 81 ppi, which will cause text in particular to look noticeably grainy.  As a point of comparison, macOS in non-Retina mode is designed around a "reference" pixel density of 100-110 ppi, depending on whether you're working with desktop or laptop displays.  A 27" 1440p display has a pixel density of 108 ppi, which will result in a sharper image and a density closer to reference.

If you only need 61W power through USB-C, then I'd look at either the U2719DC or the P2720DC.  The former display, despite being a year older, is a better display since it's from the higher-end U Series model line; the P Series is the next step down.  Both are 27" 2560x1440 displays that have USB-C inputs supplying 65W of power, an HDMI input, and USB ports on the left and bottom sides.  Neither one supports picture-in-picture or picture-by-picture.  The advantages of the U2719DC over the P2720DC are:

• The U2719DC has 4x USB 3.0 ports, and two of them support the Battery Charging 1.2 spec, which provides more power to charge smartphones and tablets more quickly.  The P2720DC has 2x USB 3.0 ports on the left, and 2x USB 2.0 ports on the bottom.  None of them support Battery Charging 1.2.
• The U2719DC has an audio line-out jack if you want to connect speakers to it, which would receive their audio from USB-C, DP, or HDMI provided by the source system.  The P2720DC doesn't have an audio line-out jack, so you'd have to connect speakers some other way.
• The U2719DC supports DisplayPort 1.4, whereas the P2720DC only supports DisplayPort 1.2.  However, if you're only working with Macs, this won't matter, because the extra bandwidth of the higher DisplayPort revision only comes into play if you'd set up a DisplayPort MST daisy chain, but macOS doesn't support that.

@wmorg  One thing I forgot to mention above.  I would recommend against a 27" 4K display as well.  A 27" 4K display's pixel density is 163 ppi, which falls basically right between the macOS standard reference of 100-110 and Retina mode's 220-220 ppi range.  The middle of the two modes that macOS is designed to use is a bad place to be.  The reason is that I'm not sure if macOS will default to treating it as a non-Retina or Retina display, but either way, it will need to either scale non-Retina mode up or scale Retina mode down in order for items to appear a reasonable size on-screen, and scaling means blurriness and scaling artifacts.  To get a proper Retina experience on a 27" display, you need 5K resolution, which is why Apple's own 27" Retina displays use...5K resolution.  So unless you specifically need 4K resolution for work you're doing, such as editing 4K video, a 27" 4K display is sort of a "worst of both worlds" option, especially on macOS since you're falling right between the two target pixel densities that Apple designs their OS to use.  (It's actually not a great experience on Windows either, but that's a longer discussion.)

Thank you very much for the insight.  Very helpful.

My eye sight is not the best (I wear special contact lenses), so a good resolution will be good.  

I will need to see if 30" or 32" might be better for me.

Do you have any recommendations for 30" or 32" displays?

@wmorg  Dell makes displays in that size, but they're 4K, and a 30-32" 4K display results in 137-147 ppi, so you're back in that problematic middle ground.  In Apple's world, the 30" display they made a long time ago was 2560x1600 (100 ppi), and their new 32" Retina display uses 6K resolution (218 ppi).  Unfortunately, 30" 2560x1600 displays are now quite rare, and the ones that are left tend to be quite expensive.  Dell has one, but it's aimed at professionals doing highly color-sensitive work, which is why it costs more than $1000 in the US -- and it doesn't have USB-C.  From a cursory search on Google and Amazon, there do seem to be some 30-32" displays that use the slightly shorter 2560x1440 resolution, but I don't have an immediate recommendation for one that has all of the features you asked about.  USB-C displays are still relatively new, so that rules out a lot of displays that were introduced earlier and would otherwise still be perfectly good displays.

If eyesight is a concern, then if possible, I would consider a 30" 2560x1440 display if you can find one.  That will give you 98 ppi density, which is very close to reference.  Even 32" might be ok at 92 ppi, which is very close to a 24" 1080p display (94 ppi).  But I think going to 4K would be counterproductive.  If you can't find a suitable display there, then I think a 27" 2560x1440 display would still be a solid choice.  Good luck!