The AA, C, D, etc. size designations have become used a little more in recent years, but a lot of people still don't understand them.
You'll find that some older people still refer to the three traditional sizes of 1.5V cells by the old Eveready designations, which were used here for many years (decades):
HP2 = D size
HP11 = C size
HP7 = AA size (also called "penlight size")
The HP actually stood for "High Power." The "Standard Power" versions were designated SP2, SP11, and SP7, so you might hear those as well.
From the same Eveready numbers, you'll also hear PP3, PP7, PP9 etc, for the 9V batteries common in radios etc.