Depends on what you're going to put on your MacBook. For example, if you have a lot of music, photos, video, files, and applications (or any combination thereof) on either Mac or Windows "side" (or both), I'd splurge for the 250gb.
Partitioning, again, depends on how much space you need for either/both Mac/Windows, be it for music, video, applications. I, for one, would need more space on the Windows side; I play a lot of PC games.
For starters, I'd say at least 100gb for Windows and 160gb for Mac.