Did you mean / rather than /boot? If you make a /boot partition, I would suggest between 20 and 100 MB, depending upon how many distributions and kernels you want to play with/
For a non-server home box, I recommend:
/ - 3.5 gig
/boot - 20 - 100 MB, you can get by without it if / is ext2 or ext3 filesystem
swap - 128 MB
/home - the rest of the drive
Proper Filesystem Layout