I would make it as big as you can afford. You need a comfort chair, refrig, microwave, tv, phone, computer, etc.
Really, I would make it 20' x 20' and if you can, make a loft at both ends to store or hang quilts from, or other crafts.
That way, if you need extra storage for your finished items, you can use both lofts. I have a small barn outback and it is 10' x 14' for my woodshop, and I would want it much bigger. Who knows, down the road, you may teach your skills, and you would need table room for that. I have made so many wood items over the years, I am proud of my work. My wooden sleighs with real runners, lined in velvet, tapestry, even a fur collar off a coat, was one of the best I done. I made a double seated one and stenciled "Over the River and Thru the Woods" on the sides. The person who bought it said her daughter uses it at the fire place all year round. Good luck in your building. Just have a great roof with tar paper underneath. I just had a shingle blow off in a rain storm and no tar paper. So water came in. I cannot take the things out in 90 degree weather so I have to wait until it cools.