I've seen scales everywhere from 6'-120'. If you can fit an axle on it, you can scale a truck. In WY they have a 70' scale, and still made me pull up twice to check my axle weights with a 61' truck, because that's the way they like to do it.
What you should worry more about is how long your customer's scales are. They may not know how to take axle weights on a full length scale, and the last thing you need on your run is putting faith in the math skills of someone paid $8.50/hr to push buttons and hand out paperwork.