When presenting your market size, VC's are looking for a vision of how big something can get. They know that 90% of their portfolio will not reach there. But they want to see the potential that you could become very big.
And so in general, you want to show a market opportunity where there is a sellable market of at least $1 Billion -- and more preferably -- $10 Billion. Go big -- you are painting the vision here of how this could become very big.
In your revenue projections, you show show a path to $100m in revenue by year 7 (if you are doing a generic SaaS business). You can tune these amounts to your business.
The reality is even the fastest growing startups of all time get to $100m in revenue during their venture funded periods. Your valuation will be a multiple of the revenue largely driven by your growth.
What may be more important here than market sizing -- if you are getting push back on the numbers you are showing -- is the pitch itself. You need to pitch a big vision. Sunil Pai's talk on pitching a big vision might be good to revisit.
For more detailed help, watch Rick Rasmussen’s talk on Financial Planning and access his slides here.