The most easy way to find out the lot size if available is to ask for a survey. Most home owners have this due to some attorneys making sure that there client is getting what they are paying for before they close. With a survey you will find out if there are any encroachments as well. Most of the time information is accurate since it comes from public sources such as the building dept. Once again though the survey is the best as this is what builders use as well.
Associate Broker, Residential Appraiser
Determined Realty Group of Keller Williams Realty Landmark
Google search NY Oasis Maps and put in the address. It will give you all the pertinent information on the property. Property Shark also gives you access to information, I think six searches a day without a subscription. I would be happy to search for you if you get me what you need.
Douglas Montgomery, Broker
This is public information and it is available at the county clerk's office. You may easily walk in there and have the tax records pulled. You may also ask the broker to provise a survey or the tax record they have on file.