How very nice, you will have fun. We have a way of going too small, but at building prices are also something we have to be mindful of too. With all of that said, look at your garage (ok, ok - without the cars and tools, Christmas tree and all that goes with it, etc....). Size from wall to wall, the take pen and paper and wright down the size sewing area you need or want. How much wall space you will need and for what (windows for natural light, wall for design wall space), ironing board area - do you like a board that stays out all of the time or do you like the kind that drops down and then folds back up into the wall or back of a door? What about cutting and pinning area? What has always been your dream size, also think about putting in on wheals (that you can lock into place). This makes it easy to move over or back (like my new center island - thought why would I need it to move - the first time I moved it when I was cooking I knew I was in love with it - that's also when I knew I wanted wheals on my cutting and pinning table). You also need to think about an easy chair or two (when you don't have to sit at a machine and if DH will come in and talk with you in your new room - same for friend or grand kids). Most of all storage - built in's - with rods (I hang large amounts of fabric (for backings, etc), you can also place a bolt of batting on a rod and move your cutting table over so the batting can be cut (then it's not fighting you or rolling away from you). Removable shelving. My dad taught me it is easier to build bigger than you think you are going to use today and it cost less, than it will to add on at a later date.
Good luck and have fun!!!!