Answer:
There are several different sizes for viols, as viol encompasses from smallest to largest: the french "pardessus de viole", treble viol, tenor viol, bass viol and violone (a couple of other variants exist, but not so common). To give you an idea, the treble viol is slightly larger than a violin, with a much deeper body, the bass viol is slightly smaller than a cello, violone is like a smallish double-bass. I've seen all sizes of tenor viols, not sure what's standard, but either way it's definitely closer in size to treble than to bass.