We live in the top of our nephew's barn! We are in a one bedroom, almost 1000 sq ft apartment that has been their guest quarters. Before I had sewn in the great room of our cabin, using the dinning table as my work space and the guest bedroom walk-in closet and dresser for storage. After moving the wife the a craft room right here across the hall? WOW!
I didn't buy new new matching things, but went the good will way also.
I really like my 4 fold down roll away tables (3'x5' open) that I can put into different configurations for cutting, piecing, sewing, and doing my own quilting. I also use the pingpong table for laying out "colors". The family spent a day up there re-organizing and left me a 3' by 6' table on which I have a lot of sorted boxes of templates, backing materials, rolls of paper, etc. Underneath I have packaged rolls of batting and bins with batting leftovers, which I use for appliques, etc., and at the end I even have a full roll of Warm and Natural. There are two big dressers, one with 4 drawers that are each the depth of 2 regular drawers which holds "patterns" of past quilts and other stuff; the other dresser holds my tools and threads and some beautiful ethnic clothes from our years in Asia. I have a 4'x8'x20"deep cabinet which houses most of my stash, along with 2 pastic 5 drawer stacks, sorted by color, of less than fat quarter size on which are boxes of sorted "strings" and "blocks", plus, on a shelf that runs above the windows that DH put up for me) about 14 matching boxes (found in a dump, all clean and waiting for me) that are labeled according to the pictorials on the fabrics, so that I can get already cut up baby prints, animal prints, little kid prints quickly; they also hold other fabrics that have some specific theme. Those boxes have mailing tape closures for opening and resealing I have two insultation boards with an old fitted sheet over them for my design area. I have a 8"x24" metal shelf that holds my quilting books and magazines. My washer and dryer are in an alcove nearby, ironing board near by tables. I use all these spaces almost every day.
The washer/dryer area is 12.5' x 12.5' (has a deep sink which I use for checking color fasness and put my fabrics in hot water for shrinking... love it!). One double window there; the wall opposite has metal shelves with basketry things that are not mine. The main part of the room is about 24' x 20' + 5' more feet when I'm using all of the ping pong table. The two outside walls are fully lined with the dressers, cabinet, plastic drawers, bookcase, etc. If it were a single room, w/o the washing area I'd really want it at least 24'x24' I love having 5 windows in my part of the room, but I'm careful about letting sun get onto my design wall or fabrics left out on tables. There are 4 more double windows on the other side and end of the room so I can really have the place open and fresh on hot summer days.
The kids are older, doing their own thing and the mama is concentrating on tri-athalongs so the family crafts have cut down or are in the house more, so this is pretty much "my" space.
If you do get a room above a garage, or even upstairs think about having railings on both sides! You don't want to have to figure out which arm to empty if you step on a toy or trip on fabric! Which brings us to the fact that your avatar has two very young people in it. If you plan to have a family that will make a huge difference in the configuration of your sewing space!!! You'll want a crib or a play area near by, and baby monitors when they aren't right there. I trained my kids to be pin hunters when they were just todlers, first showing them how sharp pins are and how important it was to NOT have them on the floor. They were so proud to help! Hope this helps.