Number of TempDB files if we can't use parallelism
Individual queries don't use parallelism? So what? If you have two completely different users running different queries at the same time, they too can make use of different tempdb files.

Of course it doesn't help all that much if you have four tempdb files all on the same volume - the real gain is when you can spread it out into multiple I/O paths.

Hard to speculate on "any slowdowns you're seeing" without more details.

Shouldn't be any issues with changing the file size. Just make sure they are all exactly the same size, have the exact same autogrow settings, and check to see if you should run trace flags 1117 and 1118.