I think you will have no problem with 40", assuming it is 1080P. I think you will notice the 1920 x 1080 vs. 1920 x 1200.
I have looked in vain for a 2560 x 1600 monitor larger than 30"
How far away are you now using the 28" monitor?
What do you want to accomplish with a larger screen?
Can you order one from a place where a return would not cost you too much?
Man I would love to have a 2560 x 1600 monitor, I haven't had much luck finding those either. Right now I'm about three and a half feet from my screen, but if need be I can get it up to five feet away. I chose the number 40 because if I"m going to upgrade, I want to upgrade lol.. I don't do any photo or video work, but I do game and watch movies and whatnot. I haven't really looked too far into the place where I would order from. I've just started the process of looking around.