Each level is 6 meters high and 30 meters across.
Think it says the height somewhere, possibly older rulebooks, but if not, it can be determined with logic (one level tall vehicles can't see over level 1 hex, 'Mechs are roughly 12 meters high and are blocked by level 2 hexes, superheavy 'Mechs can see over 2 level hexes but can't see over 3 level hexes, along with other stuff.)
The Empire State Building should be about 64 levels tall, not including the antenna which adds another 10 levels. The base is 5x2 hexes (about 130mx60m, round 130m up to 5 hexes, unfortunately this is not suitable for modeling upper floors, 5x3 base would allow for upper floors to be modeled roughly, if necessary).