I have a list with roughly 5500 items, and I would like to find out the size on disk. Is there some way I can do this? I don't mind running a query in the database, if necessary.
If you enable a site quota, an option under site settings appears called Storage Space Allocation. When you go to set a quota in the Central Administration, the page will tell you what the current storage used is so you can have an idea before there. Once you get to the Storage Space Allocation report, you can see the total size of a library.
Unfortunately, you can't get this report without turning on a site quota.