40 is too big
32 is "ok" certainly
34-36 would be nearly ideal for a desk
I'm supposing that I'll end up with some maker's 32 as that'll be the common size like the 27's of today, but sure wish they'd settle up a touch like around 34 even in 16:9 not 21:9. But wishes don't meet mass market I'm sure.
Personally I think the 40" will be the best selling one, especially if people have the chance to actually see it in action. 32" and 27" are far too small at 100% DPI, and windows has very bad support for DPI scaling.