Like so many other aspects of real estate, there is more than one standard in use here.
As a consumer, you need to know the appraiser, builder and agent all are going to use the exterior dimension of a space to come up with square footage. That means they include the thickness of walls and use out side to outside dimensions.
You will often finds the exterior space in a condo included in these numbers, so if there is a balcony , it may well be in the gross square footage.
The "unusable" space in exterior and interior walls, staircases, closets, halls and even attached garages may be in the square footage published, as this is all included in the sale and there is no differentiation of what is liveable and useable and what is just space.
Most of all, know most brokers will use the numbers given to them by the seller, or in the case of a large condo building, the numbers as represented by the builder when the units were first sold.
No where in the real estate transaction is caveat emptor more important than the evaluation of the space.
Ultimately, few buyers decide strictly based on the square footage, accurate or not. Buyers choose a living space, a location and finishes.
Most are simply creatures of convenience and use the dimensions provided by the developer (which in theory is very official) or the dimensions from a previous Realtor.
Also the square footage of the unit includes all closet spaces, walls stairs, etc. Usable Sq Footage therefore is less than actual Sq Footage.
Just let your agent know that all you care about are room sizes. You should search for higher s.f. than you're looking for and then do you own math in calculating room sizes so you get the square footage that's useful to you.
And remember to search for "0" square footage since there are some agents who don't complete that field even though they might be selling what you're looking for.
I hope that opens up some more possibilities for you.