Id do 14ft, 2 16ft w/2ft in the ground. You can do 4x6 or 6x6..but 6x6 is overkill. Gitchy some hedloks to tie it together. Id build me some little common trusses laid out on the ends of your top beam an have me a little pitched top..maybe 8 or 10ft wide..tin be cheap for that...put that bad boy on about a 3-12 an have you a bad little mammer jammer…you could form up a little pad for that thing easy too

Add an extra plug receptacle to plug in a recp saw. Also an eye in the concrete in the bottom to help with skinning.

The ones we use in Texas at the ranch we hunt the guy has a 2 inch piece of pipe buried in the ground with concrete around it and another piece over the stub up coming out of the ground. It has a winch welded to it and pulley about 10 ft high. His will swivel so you can back a truck or side by side under it and hook to the deer and just spin the deer out of the truck without having to move the vehicle. It’s very simple. I don’t have any pictures of them but his are pretty cool

Get a 120v electric hoist winch. This is different from a truck/UTV type bumper winch. They make cheap ones you can just plug into an extension cord, or wire your own 110v (GFCI) outlet. Amazon is your friend.

At LEAST 8' wide so your city friend or teenage son can pull in and out in a pickup, in the dark, without scratching paint or worse. Also, needs to have enough clearance from surrounding trees, structures, parked vehicles,fences, etc that you can maneuver. Ii think ours is at least 10' tall, and I'm thankful for the clearance. It makes it easy to put the gambrel AND scale on and just lift thee deer straight up out of a 3/4 ton. The bed of a UTV, is probably 2' lower. Love all the ideas. Reciprocating saw. Why haven't I ever used one of those?? No such thing as too much light or too many shelves/hooks. .

A pallet rack may be nice. Then you could add a roof easily.