This may be too new for any 'industry analyst' to have defined or sized well... How about taking the vertical market(s) you are targeting (ie. hotels) then showing your logic and assumptions for what percentage of that market over time will be addressed by collaborative consumption. Also maybe comment on if collaborative opens up or expands the market (so part of your number is conversion from existing market, part creation of new market).
If you are not targeting a single vertical, but instead are a platform targeting any vertical, pick the top three verticals that have or will adopt first as the basis of your next few years.
Your % adoption against verticals market size will get your working forecast to show investors - and debating the (small) percentage of each vertical I think easier than defending an absolute market size number.
You might also consider making an analogy to adoption of online commerce against traditional commerce, and what that meant for supporting tools/platforms/etc, if that helps your case..
