1/0 Bass hooks would work well for very large trout (over 3 lbs). For most trout fishing, flies range in size from No. 6 to No. 20, with most in the No. 8 to No. 16 range. The size of the fly is determined mostly by what the fish are likely feeding on. If the trout are feeding on insects and insect larvae, then smaller hooks, generally. If they are feeding on minnows, crayfish and other large prey, then larger hooks.
How large are the trout you typically encounter?