Dolly Varden: 4.5 - 5.5 mm
Lake Trout: 5 - 6 mm
Brook Trout: 3.5 - 5 mm
Brown Trout: 4 - 5 mm
Rainbow Trout: 3.5 - 5.5 mm
Cutthroat Trout: 4.3 - 5.1 mm
* Source: Freshwater Fishes of Canada, 1985, W.B. Scott & E.J. Crossman
ISBN: 0-660-10239-0
Most have a fairly wide range. Some reflective of individual size at maturity, others anadromous vs freshwater life histories. Looks like 5 mm would be a safe way to go to imitate most any. Good Luck!
Cheers,
Nog