What size bulb replaces these in a Harbor Beeze ceiling fan?
A refers to the shape of the glass part. 15 is the diameter of the base.  So A stands for arbitrary, E stands for edison, T stands for tubular and so on. The confusion is usually the base measurements. Typical bulbs are E, size of base. The small base size is known as an E12 Candelabra base.
Anything with that size, screw-in base should fit in the socket, but it has to be A15 to fit within the shade.
Also, watch out for the rating or brightness of the bulb. The Harbor Breeze model ceiling fans usually say MAX 60W. So that's around 14W for CFL type bulbs and 6W LED - which will cost the most, but are the cheapest to run in the long term.