I like the idea of one large building with a dividing wall for several reasons. If you run utilities to the shop, it's only one trench to dig & one set of conduits & pipe. One less wall to build will save time, & also save on additional concrete forms. If you aren't heating the slab, heating only half will save on heating expenses. Maybe the carport in between is okay, but I'd rather have the additional parking space enclosed from the start. It's always cheaper to build bigger from the start than add or modify later.
Rough in as many utilities as you can before you pour, even if you aren't planning on using them right away. A pipe for sewer, a pipe for water, (gas?) & at least 2 conduits for electricity, one for the main high voltage & one for low voltage like telephone. I connected a cast iron pipe to my nearby septic tank for when I eventually add a sink. I didn't run a water line to mine, but placed a large conduit with a sweep (curved) & cap down below the frost line, & out 4 feet from the wall so I can dig & slip it through later.
Electric conduit for both high & low voltage is cheap, & if you stub them up through the floor, & also to a point just outside the building with a cap, it will be there for future use. I stubbed in 2 conduits, but wish I had done 2 more. I have electric to my garage from my house, & also several sets of low voltage wires for telephone, alarm, CATV from the dish on the garage to the house. I had to add a 2nd line for CATV & had to pull out the wires, add one & pull it back through. If I had another conduit I wouldn't have had to do that.
I agree about the size, go bigger than 20 or 21 feet. I think 24 feet is probably the minimum width to have for car doors open without clearance issues. Even depth, the cars mostly range from 16 to 18 feet & to have room in front & rear to stand, jack & work is very important. If you look at the sketch below, my shop is staggered & the left is too shallow to work, but adequate to park. (I had to clear a property line & septic tank.)
