A 12” converter is 12” diameter, a 10” is 10” and so on.
If you think of a converter as 2 fans, one on the engine and one on the transmission. The larger converter has more surface area to move the car easier (lower stall). The smaller converter has less surface area and takes more engine speed to get the car moving (higher stall).
Blade count and angle play a big role in determining stall speed as well, but I think you get the idea.