Engine size and motor size
 Combustion Displacement
The traditional way of ranking the size of planes and helicopters involved the displacement (the change in combustion chamber volume) of the internal combustion engine powering them. Despite being replaced in most places by batteries, one will still often see comments like ".40 size" attached to models.
One rule of thumb is to multiply the displacement of the engine by 2000 watts to translate the maximum safe power of the engine to find the maximum safe power of an equivalent motor, with an understanding that Kv ratings / gear ratios for the latter and gear ratios for the former have to be set up optimally.
 Brushed Motors
Early on in the days of electric RC, all that was commercially available was brushed designs, and only in a few series. A company called Mabuchi set the standard for motor power with a numbered line based on the diameter (in 1/10ths of a mm) of the motor 'can'. A premium product was the 'speed' line, which modified the structure of the motor slightly but still used the same power ratings. Terms like 'Speed 300/400/450/500/540/600 size' became regularly applied to motor-ready kit planes to suggest how much power would be required to use the plane as designed.
Brushless motors with electronic speed controllers gradually outcompeted brushed motors with dramatically increased durability, and various torque ratings which mitigated the need for a gearbox. Brushless motors are rated in KV and watts rather than diameter, and so the rating has become somewhat mysterious over the years.
Due to being prominently featured in the model numbers of some long-lived early models, helicopters tend to still include the brushed motor rating in their model numbers.