How can I increase boot camp partition size?

You can remove the Boot Camp partition then use Boot Camp Assistant to make a larger partition. Or you can use Paragon Camptune X 10 that can resize on the fly.

The solution for this situation is to relocate space between volumes with Paragon’s CampTune utility. CampTune rearranges free space between HFS+ and Windows partitions and shrinks existing volumes to free unallocated space on a hard disk, and create new partitions without the need to use Boot Camp at all.

An easy alternative is to use Paragon CampTune from the beginning: resize the primary HFS+ volume, create second partition, format it as an NTFS volume and install Windows directly.

Best tool there is, and it has other features as well.

I keep telling people, 40GB minimum with 60-80GB if you can. If you want to try Windows 8 when it comes out you can maintain 7 and rollback if it doesn't work. An SSD also needs an added 10%.

Updates, page, cache, programs, temp space, hibernation - they all eat up extra space. Too little and the next set of patches and you could find it won't login or function.

http://www.paragon-software.com/home/camptune/

GPT and EFI along with Lion/ML Recovery plus Mac and Windows more than that is probably caused by having to work around fragmented partitions and free space. Also, Windows 7 & 8 normally set aside 100MB system reserved since 2009 which Boot Camp Assistant doesn't work with or understand - another reason.

Ofrten when you remove and recreate you loose some space, overhead.

CampTunes would as I said been 1st choice. It even can be used instead to setup format and install Windows without having to go through FAT => NTFS because Apple does not have the rights to NTFS.

I like a clean slate.

I use extras when they are going to do the job quicker better safer.

CampTune helps backup and install and avoids the need to reinstall (unless you need to or want to for some other reasons)