The apps you need to install are:
X11
X11 SDK
Fink
Developer Tools
Of these, DevTools is the largest, occupying about 0.5 GB I believe. The rest are quite small, but you want to leave room for all the wonderful X11 apps out there. I would budget 1.0 - 1.5 GB for the whole endeavour.
By the way, it is fully worth it. Do a search for X11 to find some excelent threads discussing the great free software out there.