Readyboost should have no problem allowing 32GB of space on Windows 7, so long as you have formatted the drive using NTFS or exFAT. If you have formatted it as FAT32, though, you'll be limited to 4 GB.
Note that you are unlikely to see significant performance gains on modern hardware, mind you. You'd be better off going for an SSD. Still, Readyboost may provide a small benefit to you, particularly if your existing mechanical drive is old and slow.