I can't help you with the actual size. One suggestion would be to measure the picture. Determine the ratio of the trailer side height to the lettering height. Then use that ratio and the model trailer to match the lettering to the model. Even if the actual height is wrong, it would be in proportion, so would look right.
Measuring the REAZ trailer on my screen (not the easiest!) and assuming the trailer is 9.5 feet high (48 inch kingpin height, roof level when attached to tractor) I got 5.7 inches. Since that seems unlikely, I would say 5 or 6 inch letters, with 6 inch the most likely.
Second try: AAR interchange rules (1977) call for 3 inch weight lettering. Measuring the "C" in CAPY and the "5" at the end of the number, almost in line, gives about the same answer, 5.6 inches. Maybe they ARE 5.5 to 5.75 inches after all.
Given the errors likely in using a ruler marked in 64ths of an inch to measure a picture, not taken straight-on, on a computer screen, that the two answers match is surprising.
