Why are there no "A" or "B" sized batteries?

There is an "A" battery. Who knew?

Cylindrical single-cell

These are all single-cell round batteries with height greater than their diameter. In zinc-carbon or alkaline types they produce around 1.5 volts per cell when fresh. Other types produce other voltages per cell (as low as 1.2 volts for rechargeable nickel-cadmium, up to around 3 volts for lithium/manganese dioxide). The cylindrical form has a positive nub terminal at the cap of the cell, and the negative terminal at the bottom of the can; the side of the can is not used as a terminal.
A 17 x 50

https://en.wikipedia.org/wiki/List_of_battery_sizes


Also see https://en.wikipedia.org/wiki/Battery_(vacuum_tube) for more on A,B,C batteries

Regarding the unanswered question about battery sizes, "B" size batteries were once relatively common. Old type "B" batteries were once used in tube type portable radios. There was also an "A" battery to power the tube filament. The "B" battery had a relatively high voltage - from 22.5 Volts to over 90 Volts. The "A" battery was a lower voltage battery 1.5 Volts to 9 Volts. You can still find these in antique shops.

http://www.stupidquestionsanswered.com/answered/battery.htm

I am old enough to remember building battery-powered valve (tube) radios in the 1950s. The batteries used therein were indeed designated "A", "B" and "C". The "A" batteries were low voltage and powered the valve filaments. The "B" batteries were higher voltage and powered the anodes. For a time "C" batteries were used to provide grid bias voltages, but were made redundant when this function was provided by other means.

These letters refer to the function, and have nothing to do with the battery size - for example, the "A" battery was usually made up of a collection of C or D size cells. The question was about battery sizes.

It's like saying a ten pound note is the same as a ten pound weight because they're both measured in pounds.

https://en.wikipedia.org/wiki/Battery_(vacuum_tube)