MSF is a must...no denying it.
As far as beginning size...go with a 500cc...
The MSF place provided some popular 250's (Nighthawks, Rebels, and GZ250's), and I learned right away that, while it was fun by default just for being a bike, 250cc's is just not enough power for a long-lasting excitement. After just two days on the Nighthawk I could feel its limitations...namely in its acceleration because it seemed to be geared very low.
Key thing is to just ride safely. Take the MSF, and then practice what you've learned on a bigger bike in a parking lot....do the slow-speed figure 8's ALOT....they really teach you how to maneuver in tight places. And practice stopping quickly without locking either wheel (remembering to use BOTH brakes at the same time to get into that habit).
So, in closing....take the MSF class...buy your safety equip (helmet, jacket, reinforced OR leather pants, boots, and gloves)...then buy yourself a 500cc. Most importantly, HAVE FUN.