What size plywood do I need to replace a ceiling in an older mobile home

3/8 plyeood will do the trick...

I would try to look at the old board to get a measurement, then go from there with questions

Would recommend minimum 3/8" thick as mobile home trusses are spaced at 24" o.c.

If your trying to do the job cheaper don't see a problem using something thin just make sure it will paint up to your satisfaction afterwards 2x6 plywood ??? LMFAO

2x6 plywood