Sometimes I use the diffusion of innovations curve for rough estimates. So what was the non-disruptive technology people were using before salesforce? What sort of customers could benefit from using Salesforce? That's an easier market size number to find, and it can be a single number (for example corporations that use some form of CRM), or a number you come up with yourself (adding different industries that you think could benefit by using salesforce).
After I get an estimate for that number, I then apply that innovators are 2.5% of the market, early adopters 13.5%, early majority 34%, and laggards 16%. I don't have a single market size number, but I have a number for 'size of innovator adopters for this technology', 'size of early adopters for this technology', 'size of early majority for this technology', etc.
Now think about market size as who are the innovators and how can you acquire them and what is the timeline, and etc for the other categories.
I could give you some of the stats in my head for a problem space I think a lot on. I don't know if it would be meaningless gibberish to you or not and what I am doing is not some proven tech or some such at this point in time.
Wave your hands really rapidly and then say: "it's a multi-billion dollar market".
One way to do it is to see how much money people are spending today to address the pain you are going to fix. Divide by 10 or 100. (You'll be cheaper, right?)
After I get an estimate for that number, I then apply that innovators are 2.5% of the market, early adopters 13.5%, early majority 34%, and laggards 16%. I don't have a single market size number, but I have a number for 'size of innovator adopters for this technology', 'size of early adopters for this technology', 'size of early majority for this technology', etc.
Now think about market size as who are the innovators and how can you acquire them and what is the timeline, and etc for the other categories.