I agree - The base install takes about 8Gb by the time you get office and the apps on its 12-15Gb - so go for at least 20Gb (I use 25Gb)
As for just removing it - yes you can but beware Vista installs a new boot loader which I prefer
for more infor have a butchers at my website
http://www.petenetlive.com/Tech/Windows/Vista/bcdedit.htm but even if you did just delete it its a pretty easy fix if XP wouldnt boot