In general, boat booms are square in cross-section, (around 6-8", as I recall) providing a walking surface for the boat crew to access the boat that has been made up to the boom. When the boom is deployed, there will be a life line to be used as a handrail, so the crew is not doing a tightrope exhibition.
On the CG where I served as Deck Division officer, we had a boom on each side, but of different lengths. They may have been 28' and 35' in length.
Beside the dangling jacob's ladders for access, there will be the same number of lines called "guess-warps", intended to tie up the sea painter from the boat.
The view from the bible looks like this: http://navyadvancement.tpub.com/12018/css/Boat-Booms-227.htm
Thw Library of Congress' Historic American Enginnering (HAER) program has some drawings of the USS Sphinx (ARL-24) a Landing Craft Repair mod to the LST
Boat boom is indistinct, little more than a pipe is shown at midships, blending with the railings. Vertical post with supporting guy wire. Deployed it will extend to 90 degrees with 2 or 3 Jacobs Ladders. Surf around the NAVSOURCE ARL images. I think there is a photo showing the ladders extended.