Normally you click on the listing you are interested in and on that first page, there is a section called "public record information." In there you will find the term "lot size." If the property has that information posted at all, it will appear there. Not all homes will have that information -- some listings just do not have the lot size on public record or it could be under dispute or never recorded. There are also times when the listing agent doesn't post that information and ultimately the listing will not reflect any lot size at all.
To make sure, if you are truly interested, you can give the listing agent or the agent's office a call to ask. If the information is available they will be able to give it to you. I hope this was what you were asking and I didn't misread. Best of luck Marilyn, it should be a pretty easy find as long as the information is available.