Horns are a fun mod, snag the supertones but if you want the max output from them you will need to make your own harness and use the relay that is included.
Stock size tire up until 2000 was 205/70/15 (or at least that is what came on my 99 outback).
You can go 215/65/15 which should be the approximate same size. People say good things about the Dunlop SP5000's.