The right size for TempDB is very dependent on what kind of system it is. BW systems, for instance, use TempDB considerably more than ECC systems, and there are a number of Notes and documents out there with recommendations on tuning TempDB for BW. Likewise, ABAP systems tend to use TempDB a lot more than Java/Portal systems, at least in my experience.
What I've found to work for a moderately busy ECC production instance with a database around 525 GB in size is to have TempDB at about 10 GB. For my production portal, with database sized around 25 GB, a TempDB of 1 GB is more than enough (maybe even the 300 MB default would have been enough, but I sized it to 1 GB anyway).
For the TempLog files, I like to start with about 10% of the TempDB size.
If you see autogrowth occurring, then consider upsizing the initial file sizes.