I think it all depends on how you plan to build your bows. What's your style? As Sam said, it's amazing what you can accomplish in tight quarters. I've seen his build alongs and I can tell you he makes a very nice bow!
If you are ordering glass and lams and just doing glue ups then you just need room for a form, oven, band saw (optional), and sanders (also optional). I've seen guys do all of the finish work (shelf if desired, handle and tip shaping, etc.) with nothing more than rasps and files and finishing with sand paper by hand. I have a 16 x 16 shop with only about 8 x 8 of working area and I make out just fine. I like to set up a folding table outside for grinding lams and running other power tools so I keep as much dust as possible out of the shop.
Bigger is always better when talking space, but remember that the bigger the shop, the bigger the mess and the harder it is to clean it when you get to where the dust is driving you nuts.