Partitioning questions

Okay, so I'm about to install Ubuntu on my new MBP. I'll probably mostly be using OS X for most things, but I'd like to at least try out the multiple desktop thing Linux has going on, heh.

Anyways, two small questions: How much space would you recommend partitioning for Linux considering the fact that I won't be using it that much, and is it possible to change the partition after it has been done? As in give Linux more space or take some spare space away?