Only gallery space is to be considered, not be confused with the total space inside the building (which is generally three times larger).
The museum should be mainly composed of human made artifacts with an important aesthetical dimension (i.e. natural history, social history, science and technology museums are to be excluded, as well as palaces whose purpose is primarily historical and architectural and secondarily as an art collection).
Museums contained inside a bigger institution cannot be considered as one entity if they are not at the same location.
For purposes of this list, large museums are those with more than 8,000 m2 (86,000 sq ft) of gallery space.