Your question, "Does this "condo" really have a 15 acre lot? Or is that the property for the whole association?" is a common question asked by buyers. And I think the other agents who answered this question were right in saying that we have to insert the acreage in the listing, knowing that the fraction that will be your share is not owned free and clear. In a nutshell, you don't get your own land with a condo.
But think in terms of privacy, if a condo association is on 2 acres and has 24 units, then the buildings and parking will use up most of the land. If there is a condo association of four units on 10 acres, then there may be woods or lawns associated with the property.
Condominiums are in fact a method to own a property in common with others, and usually the way the condo documents are written, you actually own your unit from the walls in (or sometimes the studs in), often you own the floors and doors and windows as well, and you will also own an undivided share of the common land (acres).
So if there are 10 units on a ten acre parcel, in fact you own 1/10th of the land, or 1 acre. But it's undivided, so you can't point to it--really none of the land is your own and you have to follow the rules of the association regarding land use. Maybe they will say no swing sets, or trucks with commercial tags must be out of view, parked in your own garage, or something of that nature. Sometimes they will set aside some land around your unit for your garden, so be sure to read the "condo docs"--most buyers don't read them until much later when it's too late to do anything about them. And they are mostly all different.