We closed in a garage and had it finished by a very good handyman. The shape was already determined and I had an idea where the quilting frame would go. We planned around that, but ended up changing to a different layout after the fact. I had a plug installed in the ceiling that now gets very little use. There are only two things I would change. I would install double or triple the lights we have. We had the foresight to put our lights on dimmers, but what we thought was plenty of light has turned out to be less than ideal. The other change I would make is to the floor. Since it was a garage it has a slope to it so water drains toward the wall where the door was. My handyman offered to use a leveling product on the floor, but it took days to dry and we were already needing to move things into the space (baby was 2 months old and we were tripping over stuff in the house). I'm guessing that won't be an issue for you.
Some of the things we did right: lots of plugs (every 6 feet I think), One wall of upper and lower cabinets with plugs between and a counter top for work/storage of "everyday" tools, made a cutting island out of 4 base cabinets on wheels, plug in the ceiling for Quilter, large windows, LED lights, hard surface floor for easy cleaning, but we do have one large area rug, separate circuit for a small fridge and another separate circuit for the Quilter, a cable outlet for TV, excessive insulation and a hotel-type heat/air unit that serves only that room, cabinetry has door, and also drawers for storing smaller things like blades, needles, pens/pencils, patterns, feet, etc.
I know a lot of people agonize over size, layout, and such, but I really think you cover the big things in planning, then adapt to your space as you use it and fine tune.
Have fun planning your new space!