YOUR EYE WILL TELL you that Henry Moore's "King and Queen" is seven times the size of his "Rocking Chair No. 2," but otherwise there's no way to be sure which sculpture is the bigger. To Mr. Moore size and magnitude are different things. "King and Queen" (5 feet wide and high) has the grandeur of office, the aristocratic edges of the faces assuming the dignity of the world. "Rocking Chair No. 2," on the other hand, is a toy (11 inches wide and high), made for Mr. Moore's young daughter. Its power derives from a mother and child in balance, the child standing on the mother's knees as she grasps his wrists and they rock in continuous arcs.

Both works are on display in the Hirshhorn Museum, which is celebrating Mr. Moore's 80th birthday (July 30) with a two-month-long showing of over 50 sculptures and 20 drawings. The exhibit celebrates Mr. Moore as well as the things he has celebrated: the idea of form as substance, the strength of families and, perhaps most characteristic, the seated or reclining figure, which, by Mr. Moore's hands, has been snatched from the old masters and placed in the center of modernity. The hugh, bronze "Seated Woman" seems to be trying to find a balance, an equilibrium, on her bench, even as she holds her head erect. A Moore figure sits without resting.

There's a slight disadvantage in seeing many Moores so close together, but this is a complaint about necessity, not about the Hirshhorn. In a way, the cluster works as much for as against Mr. Moore, as it shows certain changes in his ideas from decade to decade. For a sculptor who apparently changed so little over the years, he still seems to have separable periods.The "Mother and Child" of 1931, with its great sloping shoulder and implacable head - the child a feature of the mother - is much warmer and more confident than the "Mother and Child" of 1963, which looks like two dragons related by flat.

Still, the enormous "Two-Piece Reclining Figures Points" at the Independence Avenue entrance, is also a creation of the '60s, and that - while it displays, almost flaunts, a divided state of mind - seems to have come to peaceable terms with modern times. Mr. Moore has defined sculpture as "an expression of the significance of life." One may view that expression at its soundest and best currently at the Hirshhorn.