There is no special reason for a longer shank versus a shorter shank, you just need to make sure that the shank is long enough to fit your hitch thickness, and not too long to interfere with anything below it.

The main thing you need to consider is the shank diameter, the larger the stronger.

You just need to ensure that your hitch and ball combination are adequate for your Scamp.

If it were me, I would get double the rating I need for the Scamp simply because I might need to use it to pull a different trailer someday, or use the hitch on a new tow vehicle (or rented truck) to pull something heavy. Obviously, this doubling would be tempered by the extra dollars I would need to spend.

My point being that when it comes to towing, it is never a good idea to go with the bare minimum requirements. I always lean toward the heavier duty parts vs the lightest duty I can get by with.