I know this one is a little old but we run a 195/65/15 tire on my rally car and I believe my teammate is running a 175/70/15. Wouldn't go ANY bigger than a 195 unless you're making good power (think 160whp+) or are geared VERY aggressively (custom set or box).
I've run skinnier as well ~165/65/15 in a random Pirelli size.
You won't find rally tires just anywhere they're sold almost exclusively through limited distributors and rally websites. If you need help with them PM me. Also look for used sets since they'll be substantially cheaper and you won't miss the full tread if you're only Rallyxing them.
2000 ZX3 Open 2WD Rally Car