I have never done it, but the first thing I would want to know are the measurements, then the equipment you have to put in it.
Do you have windows? Lighting is something you will really want a lot of.
What are you going to do for heating/cooling?
how finished are the walls, ceiling, and floor?
Actually ... maybe I would quit thinking of it as a one car garage and consider it a long room.
I would put my quilt frame/machine in one end, with peg board behind for any supplies dedicated for that process.
I would dedicate the opposing wall for my ironing, cutting, sewing stations and make sure that everything (cutting and ironing anyway) was at a counter height - - table height for cutting and ironing will really tear up your back. I think that most cabinets are between 34 and 36 inch heights.
And, I would plan a LOT of cabinets for my fabric stash.
Part of one wall for the peg board to store my rulers, my tools, my cutting mats ... there are a couple of threads going that have people making their new sewing rooms. You might find their sewing rooms inspirational.
If you have access to a salvage yard, you might find all the cabinets you need on the cheap from an old travel trailer ...
and, LOTS of plug ins ... it will save you running thousands of cords for all the little things we figure out to plug in, and you just never know where a plug in will come in handy - - in a long room, two per wall probably is not enough.
And, I would put in a banque of drawers. I would probably put my threads, my tiny tools, my pin boxes, pin cushions, the types of things that won't hang up into those drawers.
Keep us in the loop for what you are doing, and how you are accomplishing it.
New construction is ALWAYS a wonderful thing!
And ... if you don't have a quilting frame/machine, but you like to do free motion quilting .... built an extension table on to the table your machine sits on - - make it almost four feet wide, and about three feet deep ... put it in the corner of your room, so that the wall is running on your left side and in front of you as you sit at your machine.
Being able to "trap" your quilt into that area, your free motion quilting will be much easier ... your machine will sit on the corner of the whole set-up as far right and forward as you can get it.
At the factory, when we were working at our machines, not only did we have that extension table (even doing chain piecing can benefit with that set-up) We had a smaller table that sat on one side of our chair or the other ... it was a bit higher than the seat of our chairs. This is where our pieces sat as we worked through the packet.
Oh! And, don't forget a place for your radio, CD player, and television ... it is always more fun to sew when you have some good country music or a quilting program on the tele.
This is so fun!