I have a similar setup, except I am triple booting, Vista, Backtrack and OSX. I just installed Backtrack (i'm posting this from Konqueror) I had to install several times to find out an amount of space that would be appropriate. I eventually went with a 17G partition. Like you I only plan on using backtrack on a fairly rare basis, so I didn't want to waste any space.
With an install, and then a safe-upgrade with aptitude from the backtrack servers, I ended up with 4.7G free. Also, and I'm not sure how much it matters, I am using the x86 version of the OS. With all that in mind, you can figure where you want to be based on your anticipated usage patterns, as noted from a previous reply, if you choose to install open office or whatever other "Producivity" software you might want. Just a note though the safe-upgrade sent my free space somewhere in the 3G range, but that was only temporary, and the space came back after aptitude was done.
I realize your post is a month old, but I hope this will help you or anyone else with this question. I ran across this from google, so i imagine others might be lurking who needs some help.