I do a lot of KATY Trail riding, which is crushed limestone. What everyone else says here is good advice. The wider you can go, the better. 28s or 32s will make life much better, and if you can go wider, you'll enjoy it more.

If the trail is wet, wider tires don't sink in as much. And if the trail is baked, the wider tires will absorb the bumps of the irregular surface.