Sounds like you are after a hard car cover more than a work space. So I will go off that idea instead of the typical workshop response I see.
In my local area I can do up to 199 sqft shed without a permit. Go larger or add utilities, permit is needed. Check your local area. For me I would do 10x19 just to skirt permits.
If I had to go permits no matter what, 10 or 12 by 24. The width is a bit large until you think about shear loads. Do an 8' wide door opening and you have very little shear (keep the garage door from racking) even with 10'. My choice would be 12' and offset the garage door a little to one side. Say 3' on one side and 1' on the other. The 3' will offer better shear loading for the structure. And better access to get in/out of the vehicle. And better storage for other things.
Speaking of storing other things, at the front of the garage I would do an oversized man door. 42/48" wide. This would let me store and access stuff in front of the vehicle. Lawn mower, wheel barrow, etc. This is also why I would stretch the garage to 24'. Even later if you place something besides the Bronco in there there will still be room. Some larger vehicles are pushing 20' mark. 24' will let you park nearly anything in there and still have room for other things.
The smallest I would go for is 10x20. You really can't park anything you care about in less than 10'. You better be skinny to squeeze through the door. Less than 20' and you are severely limiting what the storage is good for later. Go to sell the house, hard to call it a detached garage when it is too short to store a normal vehicle in.
Even if you are not conditioning the space, insulate it. Use an insulated garage door. Insulation in the walls. Does wonders for basic thermal managment.
The thing I have against most of the pre-built tuff shed type storage buildings is the wood floor. For vehicle storage I much prefer a slab.