I'm a visual thinker, so I tend to mock stuff up. Whenever I've faced a need/desire for a new storage structure, I gather as much of the stuff I want to put in it together, and see how much space it takes up. Can you get around stuff to get what's at the back, or do you have to move everything to get at anything? Can you double stack anything? Is it strictly storage, or will you use the tools your wife is kicking out of the garage?
Keep in mind that the electric cost will likely be increased by the cost of bringing power out to it, possibly needing increased service to your house. Also, if it goes through a separate meter, I think Michigan law allows the utility to charge commercial rate for a non occupied dwelling - I was paying almost $30/month in readiness charges, and about $3 for electricity. Check with the utility. How much of the wiring can you do yourself? On my new barn, I had real electricians bring the power in, install some overhead lights, and start me a couple 110 circuits, which I extended to many more outlets. They are coming back to put in 220 in a few places. I also dug the trench from the meter on my house to the barn.