It would be so much better if you measured the space vs. sending me to a website description but I went and I think I interpret the description to mean you have 28" of width between the vertical rails/posts that hold the shelves and probably 24.8" of space from desktop to shelf above desktop.
So then I looked up a random 24" monitor on Amazon and a 24" monitor is 16.5" tall and 21.8" wide. So you would probably have room for larger than that in both dimensions if my interpretation of the space is correct. Again, you should measure so you know for sure.
That 40" I chose could easily fit inside the vertical space of up to 24.8" if that is correct. However, it is an ultra-wide and 37" wide. Are those vertical bars far enough back from the front of the desk such that a monitor could be in front of them and yet be far enough back on the desk to be OK to you? Or would that be too close to the front edge for you? It's hard to tell from the picture but they look pretty far back, so maybe. You might want to cut out a piece of cardboard 37" wide and stick it there to gauge how close the screen would be to the front edge and also consider the left & right overhang too (towards 5" overhang on both sides). Having some space behind is OK. The stand needs some space back there and you can always tuck accessories (more storage for one) and a desktop back there too.
If all that works well for you, mine is
Dell 40" 5K2K Ultra Wide, typically priced about the same as ASD. If you determine that overhang left & right is just too much, you can work your way down several sizes there to shrink the width. For example,
maybe 34" which is height on stand (adjustable) between about 16" and 21" (easily fitting within the apparent 24" height "hole" you have and width is 32" which would only have about 2" overhanging on each side of the 28" space. Again, maybe fold the cut-out cardboard mockup to that size and put it in place to see what you think of the dimensions of that one too.
And, of course, Dell is not the only monitor maker in town. There's
plenty of them. You could do either ASD clone from Samsung or LG... or ASD itself if desired. They are all 27" monitors and should all easily fit inside of an apparent 24" vertical "hole". For example, ASD is advertised to be 18.8" tall on stand.
In all of these options, you might want to also consider a stand RISER if you have the space. I have one that is just tall enough to slip the keyboard under it when I'm done using the Mac and it has a couple of easily accessible USB ports. If you get one not too tall, you can probably get similar utility that fits the "hole" too. Just hop on Amazon and shop for "monitor stand USB hub" and similar and there are MANY choices. Then just do the math of monitor height with stand + riser height to be sure both come in below the apparent 24" total height you have available.
Lastly, if you want both but the monitor stand isn't height flexible enough to make a favorite riser work (too tall in total), most monitors come with a VESA mount option, so you could get a VESA mount to put into the wall to make the monitor "float" and not use the monitor's stand at all. You would need one that will let it hang far enough away from the wall to get it in front of the units vertical rails/posts but there are plenty of those available. Then you could have Mac, keyboard, mouse, perhaps any kind of hub (monitor riser or not), other storage, etc below and behind the floating monitor.
