I'm dual booting Ubuntu 12.10 with Windows 7 & I installed Ubuntu 12.10 on a 100gig partition but the biggest HD space option when I installed Ubuntu was only 30gig. How can I allocate more space for Ubuntu on that partition?
I'm assuming that you were able to see an empty 100GB partition from Windows. If that's the case, Ubuntu setup should be able to see the same 100GB partition and you should be able to allocate the entire partition to Ubuntu.
Your best option here is to run setup again, and wipe the partition during setup. Use the manual partitioning option ("Do something else")