My win partition (win7 64bit ult) is current 55.7GB used
I try and make sure nothing except Antivirus in on my OS drive (its 120GB SSD.. so 111GB usable)..
but I don't go totally anal about it..
I don't have a hibernate file..
Most of my iTunes and word/excel docs are not on my OS drive..
So if you go anal.. could go 40GB
Go like me.. 70GB
Be more lax... 150GB
Does not know how to stop apps installing on C drive...don't partition.
make it just a touch smaller than the SSD that might eventually go in its place. so, up to 120G is fine since that gives him the option of a 128G SSD later on. or go a bit bigger and partition it for 230G to give a 240/256G SSD option down the road.
personally i have 128G set up, and Win7 takes up about 50G with all the apps and drivers and whatnot i have installed, though that includes about 3G worth of docs that i have to relocate to a separate drive