Welcome to the forum!
There are lots of things to consider, especially your desk depth and your distance to your monitor. Personally, on a desk, I wouldn’t go further than 34" 1440p or 2160p (the later with some scaling). I sit at about an arm’s length.
I used to sit at 10-13ft (3-4m) from the screen, so I had a 49" 4K TV, which I had to scale to 125-150%. I’ve moved and downgraded from my TV to a 15.6" 1080p portable touchscreen monitor at 100% scaling. I’m happy about it.
But in your case, if you use Linux most of the time, then 1 big monitor would be easier to manage if you have a tilling WM. Well, managing 2 monitors was always a chore for me, which is why I always preferred having just 1 bigger one than 2 smaller ones. I make use of workspaces, so I can keep a few things full screen and other things tilled.
When you introduce scaling, text sharpness becomes an issue. I’m blessed with good eyesight, so I don’t really care, but it is bothersome for some. A 4K 40" monitor would probably be pretty sharp, but it’s probably too small (unless you sit very close to it), so you may need scaling. As others pointed out, a 43" 4K panel would be ideal depending on your distance to your monitor. And it should give you a little more space than just 2x 1440p monitors.
Some people swear by ultrawide 3820x1440 monitors, but I think you are sacrificing one tile just to have every window in-line (i.e. instead of 2x2, you get 3x1 tiles on your screen on ultrawides), so I believe a 16:9 / 16:10 aspect ratio to be a better choice to view more content. However, I can understand the appeal of not having to raise your head to see the top corners. I tried sitting at the desk with my 49" TV and the top was hard to look at (from an arm and even 2 arms length).