BLDC motor design, physical appearance question
This really depends on the application. 
Servomotors are generally longer than wider and this is in order to keep moment of inertia as low as possible, which increases linearly with the length of motor and with a square of radius. This is where you need dynamics.

For other applications, where top-notch dynamics isn't required it is preferred to increase diameter, since output power (and torque) increases with the diameter squared. With larger diameter the slots are bigger, therefore electric loading can be increased by the number of turns or number of parallel branches to decrease losses/increase current. The length also increases power but linearly as well as power losses both in copper and core.

In series production, it is favorable to pick a basis (given diameter) and increase length in order to achieve higher power and torque ratings. There is also an upper limit to how big the radius can be, given that centrifugal force increases with radius of rotor and with square of speed, so your magnets won't fly off and damage the motor.