What is "normal" SBS2003 WSUS file size?

I’m a small business owner and have a server that was set up by an IT guy that unfortunately isn’t around anymore. He set up the server w/ the OS on one 25GB partition of a 225GB hard drive. I keep getting error messages that the partition is full. Can I safely move parts of the OS over to the other partition, if so which files? It seems that most of the files are located in Windows (4.5GB), Program Files (6.9GB), and WSUS (9.9GB). If I understand, WSUS is for automatic backups (?) and if so should it necessarily be so large? Also, is resizing the partitions feasible or reasonably safe to do?

Thank you!