It wouldn't be appropriate to use the scale tool nor the Tape Measure tool on the trailer at least in one go because that will resize things you don't want to resize. You're better off to use the Move tool for much of it. You could scale the main frame but things like the lights and undercarriage will need to be moved, not scaled.
As for scaling the main frame, the Tape Measure tool would work however if you double the width, you'll also double every thing else. That won't get you a 20' long trailer. Stretching its length by another 4' will wind up deforming a lot of the frame members, too.
There are enough dimensions that will get changed inappropriately in this model that I would be inclined start over with the frame. Or, if you don't want to do that, there's another trailer model in the Warehouse that is 8-1/2 feet by 18 feet. It is actually built so it can be modified more easily and has an appropriate tongue and undercarriage for a trailer of that size.
Both models need attention to some details stuff like face orientation.