You are correct. We had a 12' popup which referred to the box size. Now our 13' scamp
is actually closer to a 10' "box" length and 13' total hitch to rear bumper. I have not found any exceptions to that general rule among the fiberglass trailers yet, but others here have a lot more experience than I have.
We also switched to a fiberglass from a popup recently. I hope you enjoy it as much as we did. If you have questions or want to share experiences, just ask. Welcome to the forum!