If possible make it the size of a nice 'guest house' complete with bathroom and a small fridge and microwave.
Remember lot of natural lighting is welcome if you can do it. also it is nice to have air flow on those wonderful days when you have your doors and windows open.
My quilting machine is out to 12 1/2 ft and I need more than 4 ft around it, as it takes up that much space, and if I want to get around it I have to move it. Not fun.