Bryce Roberts (OATV) totally nails it in his post titled You Can Never Size a Market in Excel. The punch line is:
“In every single case that I’ve seen this question (“how big can it get”) persist over more than 2 meetings, it has NEVER been resolved. NEVER.”
There’s plenty of great stuff in the post – go read it.
Remember – Ask the VC reads all of the VC bloggers for you so you only have to read the best posts!