Use the various suggestions below to estimate your market size. Do both top-down and bottoms-up. Then, for both estimates, consider what would happen if you were off by 10x (in a bad way - you'd overestimated your market size by an order of magnitude).

Do you still have a viable business? Yes? Great, go for it and stop worrying about market sizing! No? Revisit your assumptions in the market sizing calculations - if they are as good as you can make them, and you find that being off by an order of magnitude will kill your business, then proceed carefully, because the actual market size may in fact be 1/10th what you originally estimated.