It's hard to find rental comps on vacation rentals. On AirBnb, it doesn't list square footage unless you contact the owner and ask. And how to estimate vacancy?
Try using AirDna to get more in-depths stats about STRs in your area.
I would not be too concerned about square footage of vacation rentals. The number that is most important to renters is the number of bedrooms. I have found that renters don't read much of any listing. They mostly look at the photos, which need to be top quality.
I would look at current rental listings in your area on Airbnb and on HomeAway/VRBO and look at their nightly rates and look at their calendars. You will learn a lot about your rental market by doing this. I am in Orlando, and I have found that the short term rentals that are either new or that have been renovated and that offer the best amenities (i.e. - game rooms, movie theaters, themed kids bedrooms) rent for higher nightly rates and more often.