Effectively all of it, which is not true of every warehouse building.
In a column-supported structure, every column takes floor area out of use and, more significantly, dictates where racking runs and aisles can go. The practical loss is always larger than the footprint of the columns themselves, because the layout has to accommodate them. A clear span removes that constraint entirely, so the gross area and the usable area are close to the same number.
Length is the flexible dimension. Buildings are assembled in bays, so a warehouse can be extended by adding bays to one end later. The span cannot be changed once the frame is manufactured, which is why the width decision deserves more scrutiny than any other measurement on the quote.
The practical approach is to lay the racking out first and let the building follow. Work out the pallet positions, aisle widths and pick faces you need, then size the span and length around that. Sizing the building first and fitting racking into it afterwards is how usable capacity gets lost.