Some folks include the whole area, but technically the sloped ceiling height should be 5 feet to be included in the GLA. Also the whole area should have at least 50% 7 ft wall height. This is taken directly from ANSI at
How to Measure a house
"Level ceilings must be at least 7 feet high, and at least 6 feet 4 inches under beams, ducts and other obstructions. There is no height restriction under stairs. If a room has a sloped ceiling, at least one-half of the finished floor area must have a ceiling height of at least 7 feet. Otherwise, omit the entire room from the floor area calculations. If a room with a sloped ceiling meets the one-half-of-floor-area-over-7-feet requirement, then include all the floor space with a ceiling height over 5 feet. "