I have a 49 key. Wish I had a 61 key version of my board, but I didn't have the money or space.
I get everything done on my 49 key. It just gets annoying when having to change an octave to play a piano part from low to high (but you get used to hitting the octave button mid performance). The only other real annoyance is it's difficult to use keyswitches for sample libraries like Mojo Horns and Garritan and other Kontakt stuff that use them a lot.
So ultimately, I wish I had a 61 (and a separate 88 key for full piano feel and no restrictions at all), I get work done on my 49, and I couldn't imagine going lower than 49. If I were on an airplane, I would either grab a 37 key, or just a pad controller. I hated my 25 key... got rid of it.