I am limiting this to 1440p (2K) monitors, since it's what I use.

My opinions:

1 a 16x9 aspect monitor in size 34", 2560x1440 gives too narrow of a view.

2 a 21x9 aspect monitor  size 32", 3440x1440 gives a better angle but the vertical size of the screen is too short, less than 17".

3) Combining two 34" 16x9 into 32x9 aspect will give 5120x1440which is too wide because MSFS will elongate (distort) the images at the edge of the screen quite a lot. But the large image is breathtaking.

4) Combining two 32" 16x9 and then shrinking (dragging with mouse) the MSFS image to be about only 2/3 of the way across the combined screen will give much less distortion than double wide. And compared to 2) a much larger image vertically and horizontally. And there is less distortion than 32x9. But there are 2 bezels side by side running vertically down the image. The  other 1/3 of the combined screen image on the right, can be used for charts and such. And they are right there in front of you almost.

There's a way to eliminate bezel distortion in Nvidia control panel (called bezel compensation). But it still leaves the black bezel stripe down the image, over on the right side. But at least for instance a round gauge behind the bezel will stay round, it won't look lopsided.  **

5) Best of all these are the 49" 2K (1440p) double wide monitors. These are precisely and exactly like two 34" 16x9 side by side. There is no vertical bezel stripe. Drawbacks: the monitor is large and heavy and unwieldy to move and lift. Look on a site for instance like Amazon for a couple dozen different monitor mounts. Some attach to a wall behind the desk. Others work with a C clamp that clamps on the back of the desk. Some of the more expensive ones can hold a 40 pound monitor.

 

** Bezel compensation.

run Nvidia Control Panel | Configure Surround, PhysX

check Span Displays with Surround, click Configure

In Topology choose 1x2

In Displays checkmark both of your two displays

Drag the picture until screen 1 is on the left instead of the right if necessary.

Click Enable Surround (Do this right now). This will give you other resolutions to choose from, namely double wide.

In Bezels, click the up arrow until the image aligns (I needed 42) . Don't enable surround again yet.

Choose Refresh Rate from drop down menu.

Now click Enable Surround again. (this will give you even more choices for resolution, namely the bezel corrected resolutions).

=> Under Resolution pick for instance 5162 x 1440 (the span will work but the bezel correction will not work if you leave it at 5120 x 1440).

Things may change with FS2024, might be a no distortion at the edges view for wide screen display settings.

I  haven't seen a double wide 4K monitor, but  there are wide view 4K monitors which require driving a lot of pixels. But a 4090 should handle it. Perhaps it won't take a 4090 to do that in FS2024.

Dell/Alienware sells a 38" diagonal (35.2" wide) 3840 x 1600 display that improves vertical size over the 34" 21:9 displays and doesn't involve the hassle of bezel management or the performance hit from driving multiple monitors. 

It sports G-Sync Ultimate, too.

I see it was on an Amazon $200 off black Friday sale too last month. But not anymore, 😞

I bought that monitor (Dell/Alienware AW3821DW) on Amazon for US $720 ($380/35% off) after cyber Monday a few weeks ago.  I see them on sale for ~$900 semi-regularly, but that was the deepest discount I've seen to date.

So far I'm pretty happy with it.  I had an older 34" IPS version of the monitor on my MSFS/XP machine before (it's now on a separate machine for shoot-em-up and driving games), and the 38-incher is just enough larger to where I can read all the displays on a Boeing or Scarebus MFD without having to crane my neck in closer.