Steel tubing normally comes in 20' lengths. For the 1.5" square tubing used to make DIY No Weld Trailer Racks, some places will also stock it in 24' lengths.
When purchasing steel at a steel yard / supplier, normally the cost per foot will be less if you purchase a full "stick". Check the full "stick" price verses getting just how much you need. It isn't uncommon to find the per foot cost for a partial length to be double that of the per foot cost when buying a full length of tubing. In many cases, getting some extra, will end up being less expensive.
Here is an example of how to maximize your material usage on a No Weld Rack. For a 4' x 6' utility trailers, you can do Medium Height Cross Bars with one 20' length of 1.5" square tubing. If you make the cross bars 63" wide, which is slightly narrower than the fenders, the upright would end up being 28" tall, leaving an inch or so after cuts.