I use the late 2011 dual i7 13" macbook pro model.
updgraded the RAM to 6gig and performance is fine.
Screen size is alright, i do run dual monitors however in osx lion you can just create more desktops and swipe between screens (sequencer on one desktop, mixer on another etc)
I bought the 13" model for more of a hybrid studio & DJ performance laptop.
I'm happy with it.