For the last few weeks I've been watching these flowers open on the tree beside our side deck. Today I was finally motivated to measure them. They seemed larger than usual. I convinced Peter to pull a branch over to the railing and we used a yardstick.
According to Dirr's Manual, Magnolia macrophylla flowers range in size from 8-10" occasionally reaching 14" across. The camera is not perpendicular to the yardstick, but I was there and that flower measured more than 20" across. I expect that the cells in flowers, like those in leaves can, expand to larger or smaller sizes depending on the amount of water available s when they are maturing. We have had plenty of rain this spring!