A 40 gallon breeder is about as small of a tank as you want to have for a Musk. They're actually very active turtles and they'll take advantage of all the space you can give them.
I currently have a young musk turtle in a 20 long, and depending what my money situation is in the next year or two, he will be upgraded to either some bigger tub or a paludarium I've been thinking about trying out in a 75 or 90 gallon