You can use disk management to extend a partition to make it larger, if there is unused space behind it. Then the partition would be large enough for whatever.
We are talking about one hard drive here, or do you have 2?
When you do a custom install over another OS, the drive is basically wiped, but the old files are moved to a folder called Windows.old and a couple of hidden folders. So it takes a little extra room, about 2 or 3 GB for the old files, depending on how much stuff you had on the Vista install.