Answer
Monitor the TempDB for a week, and then create two files that are 75% of the maximum size it reached over that week.
Shrinking the tempdb is a bad idea, as you end up fragmenting the physical file all over the disk. It is better to allocate the file(s) once with sufficient space to grow.
The following article provides some detailed information about how to optimise the TempDB:
http://sqlserver2000.databases.aspfaq.com/why-is-tempdb-full-and-how-can-i-prevent-this-from-happening.html