Allow windows to have 10-20Gb fat32 - this is generous for XP-home unless you do lots of raw music/video editing (even then, linux is better for this) - and leave the rest for linux. Linux does tend to occupy more drive space that windows because it is a modular system, and it includes many more applications by default. Still, 20 linux gigs is fine for a trial.
Also - looking at the way you use windows, I suspect you will tend to use Ubuntu mostly and windows only occasionally. I know it dosn't feel that way now - you wait