I have worked on everything between a 11" Macbook Air and a 17" Macbook Pro. I recently switched from a 13" MBP to the 15" one. Both had 16 GB of RAM but for my work, the quad core CPU that is only available in the 15" models made a noticeable difference and I'm happier now.

Portability: I travel a lot and the 15" is too heavy but the 13" Macbook Pro was heavy as well and in combination with everything else I'm carrying around, the difference doesn't really matter. I got a nice bag that I can convert to a backpack instead. The 15" doesn't feel very "clunky" to me. I was worried that I wouldn't want to sit on the couch with it like I used to with my 13" but here I am and I like it.

Screen size: the 15" screen is a little bit more convenient but the 13" was no problem for me either, you get used to it.

I would never do 11 or 17 " again, the 13 and 15" are both ok to work on.