It depends if you are talking about a co-op or a condo. You will not find square footage in public records for a co-op, because in a co-op you own shares in a the corporation that owns the building. You do not technically own real estate. Those shares entitle you to a proprietary lease for the apt. In a condo, all past sales that are recorded will show square footage in the ACRIS system (city real estate recording system which is now online). As far as new units, the offering plan will indicate the square footage of each unit in the building.
Hope this helps.
Halstead Property, LLC