If you're going to have multiple documents open at once (ie. side by side) or you feel the current iMac is lacking, 27". Otherwise, 21.5" would be fine. Personally wouldn't go with a display smaller than 27" now, but that's just me.
Seeing as you are considering the i5 with the 27", why not go for the i5 with the 21.5"? The i7 may not be worth the premium in this case, and both will be significantly faster than the old machine.
As for the MBP, unless you need the portability, there is no point paying a premium for it.