My serial number is N565MS1504 so I guess that makes it a 565.
Thorn is very unusual in one regard, they size their bikes by effective seat tube length like all other companies. But they also size them by top tube length, usually you want a shorter top tube if you fit the bike with drop bars, longer top tube for flat bars. The top tube sizing is usually specified as small, medium or large, or some combination. For example my Nomad is a 590M, the M is medium top tube length.
If you can compare your frame dimensions to the catalog that was in effect when it was new, you can use a tape measure to figure it all out.