I had them made 6 years ago for that trailer, which is lower than most at back.
Hugh did engineering math for me, needed 11'4" to not scrap loading,
I had made 12' to be safe, work great.
I think they would fit under car, but never had to.
I load in back of box, up in air over tailgate, with trailer on
no overhang issues, heavy enough, not going anywhere, but
I use tie down straps at back to keep in place and has worked great
for my uses so far. I also have a custom 6'5" set ( longer than originals)
for other uses if needed.
MY trailer is Aluminum 10,000 lb trailer, 22'long wooden floor
but fairly light weight. WOrks great for hauling any car or truck.
I have since added a 4000lb winch, makes it good for moving aorund my other projects
Keep in mind my R in stock form is lower than most cars.