Quite similar to @ABrandt, I estimate the market using the stats of website visitor from compete.com/quantcast/alexa, etc. E.g. let's say my nearest/closest com

Quite similar to @ABrandt, I estimate the market using the stats of website visitor from compete.com/quantcast/alexa, etc. E.g. let's say my nearest/closest competitor(if any) get around 100K visitor per month, let's say I get around 20% of his traffic, and if 3% these traffic convert to be customers. I get a high level figure of the market size for my products.