Here is a pic of the garage ceiling in a house I built, which had living space above it.
I had an 8 ft and 16? ft door, and a entry door as well. I can't remember the dimensions, but was at least 24' deep and 30' wide.
I am not sure what those 'structural' beams/trusses were called, but I did not have any posts in the garage. They were probably expensive, but I was building my dream house lol and didn't care.
And the living space was not a trampoline.
Haven't seen it mentioned, but I would consider an entry door as well. Not a requirement, but something to consider.