guestimating from the pictures at hand, I'll back up subfixers guess ...
Tha sailor in the hatchway of the mule. Assuming that the hatch is roughly 6 feet tall - the beam is just over 2 hatches wide -- call it 15 feet.
See the magazine ad. Assuming that the beam is 15 feet wide -- the length is about 2 beams long -- call it 30 feet.
You're talking about something which was similar in size to a LCVP -- 36 feet long.
A LCM-3 was 50 feet long. A LCM-6 was 56 feet long.