How to find ARV when house sqft is unknown

[Christina Jean-Francois]

I have just started wholesaling in my community (Suffolk County, NY) and I realized that for most single-family homes, the county does not have the house sqft recorded. Only the lot size. I am using the wholesaling with an agent approach and they won't know the sqft either but will mention that the house is on a "so by so" lot and priced accordingly. Does anybody have any experience calculating ARV by lot size?

When I go to check neighboring houses that were sold I didn't see any correlation between the sizes of the lot and the price they sold for. 

How can I properly calculate my ARV besides the comp reports I get from the agents?