What do you mean by that, what's the largest partition size a FAT32 can be? Or what's the largest contiguous file size it can hold?
Windows XP's formatting utility can create a partition up to 32GB, if you want larger than that you'll need a program like GPartEd or PartitionMagic.
The largest file size it can hold is 4GB minus one byte. So if you're planning to use this FAT32 partition to store DVD ISO images, stick with NTFS or use a UNIX file system like EXT3 or ReiserFS.