my Win7x64 install with a handful of installed apps would have fit onto a 64G SSD since it only took up around 45G. but with price differences between 64 and 128 being quite small ($65 vs $90), i myself went with a 128G.
but you should be just fine with a 64G. you'd have less unused space for the system to shuffle files around as it needs to (for wear leveling), but i'm guessing even that would be perfectly fine for 2-3 years of use.
just make sure that you MyDocs gets kept efficient or is redirected onto a separate HDD, and same goes for MyPics and MyVids, and make sure you regularly sort through your Downloads folder to move stuff off the SSD and onto the HDD where it belongs.
and don't keep a ton of crap on the desktop. but...