There is no standard for exhibitions or museums.
In all cases it is the photographers choice.
For me personally, I always print 344 x 277mm on 40 x 30cm paper. This is because I can then reuse the 50 x 40cm window matts for subsequent exhibitions. I have also found that this is a popular size with purchasers because a framed print size 50 x 40mm is not too large or small for peoples homes and (on the rarish occasions a professional collector has bought one of my images) a properly mounted and matted image size 50 x 40cm is something that can be easily stored in the metal plan chests that they all seem to use.
Bests,
David.
www.dsallen.de