Are you talking about CRTs or LCDs? I'll assume LCD.

Personally I would recommend 20" 1600x1200 monitors, but some may find 19" 1280x1024 more comfortable for their eyes (larger dot pitch). Then again for games it does not matter. DOS games will feel more at home on a 4:3. 17" 1280x1024 I would not select, unless there is a shortage of space. Whatever you do, try to get an IPS monitor, to a TN one. VA would be OK, but some of them are still slow and you can experience ghosting in games.

Most 1600x1200 20/21" displays are not capable to produce 70hz without frame skipping, which is required for DOS.

4:3 resolutions in that range are 1024x768, 1152x864, 1280x960, 1400x1050, 1440x1080, 1600x1200.

I used to use mostly 1152x864 on a CRT (which was capable of 1280x960 but distortions were higher and text less readable).
Now I have 1600x1200 LCD (Dell 2001FP) which is great.

I would never recommend 1280x1024, that resolution was an abomination. For me any amount of stretching is unacceptable.

I need to save space so a CRT is a problem Ill go with an LCD, So do I need one with a refresh rate of 70hz or more for Dos I thought 60hz was ok I spose it will run smoother on a 70hz+

The Serpent Rider wrote:

Most 1600x1200 20/21" displays are not capable to produce 70hz without frame skipping, which is required for DOS.

I have a Dell 2000fp and a Dell 2001fp. They are both 20" 1600x1200 LCD monitors. I use them both on Windows 98 / DOS machines and have had no issues with frame skipping.

IPS and VA panels normally can't display anything beyond about 64Hz, so monitors have to work around that with frame skipping. There might be exceptions of those panel types that can properly handle faster rates, but I don't know them.
Since the video modes typically used in DOS run at 70Hz, you would need a TN panel to avoid the skipping problem. However, this problem isn't as apparent if you don't run things that have smooth scrolling.
1600x1200 TN panels exist but they seem to be the minority.

This thread has reports on a lot of monitors. Samsung seems to be pretty good about having an option to crop the image to 4:3 on their widescreen monitors, but from what I've seen, *not* on their 5:4 models.
Widescreen monitors and 4:3 aspect ratio compatibility thread

The 5:4 aspect ratio and "1280x1024" being a standard resolution always drove me crazy. It didn't fit in with the rest of the standard PC resolutions and aspect ratios. Don't get me started on games that only provided options for 800x600, 1024x768, 1280x1024 and 1600x1200. Arghh... 🤣

I used CRTs exclusively until 2010-2011 and actually went back to a high end CRT for a while in 2015 before buying one of the most CRT-like LCDs on the market (BenQ XL2720Z).

For old games there's just nothing like a CRT... and almost every one of those is 4:3 (except for the Sony GDM-FW900 and maybe a few others).

In short, I'd go out of my way to get something that was 4:3 if I was the kind of person that put a lot of money and time into retro-computing... I still have my big huge CRT, I have two more decent ones storage, plus the IBM 5153 CGA CRT on my 5150. The only thing I use a 5:4 LCD for is a basic screen for testing computers because I don't want to waste the lifetime of my CRT with hours of DOS prompts and stress tests... I still have to use the CRT many times though because lots of older systems don't get along with my 5:4 LCD.

The 5:4 aspect ratio and "1280x1024" being a standard resolution always drove me crazy.

The interesting factoid is 1280x1024 resolution and 5:4 aspect ratio was linked together only by the LCD industry in the LCD era. 1280x1024 existed as a standard VESA resolution long before the LCD boom but just like 640x400, 320x200 etc. was considered a non square pixel resolution intended to be aspect ratio corrected to 4:3 just like the others.

Some monitors seem to have trouble distinguishing 1280x960 from 1280x1024, like the abovementioned Dell 2001FP when using VGA connection.

Since 1280x1024 is more square than 4:3, the monitor pillarboxes this resolution for correct 5:4 aspect ratio. But it does so also with 1280x960, which is then letterboxed, resulting in black frame on all borders (even in "fill" mode which is supposed to stretch everything). This does not happen for me when using DVI.