Connect it to a 12v supply such as a car battery and add an ammeter (in series).
Measure the current.
Now load the motor by trying to stop the shaft, but down't fully stall it.
Now measure the current.
Find an online catalog of 12v motors.
Plot motor volume vs. current draw on whatever kind of axes gives you a straight line (linear-linear, log-linear, linear-reciprocal, Weibull-linear).
Use your graph to predict current draw.
So the correlation coefficient for
motor volume and current rating
for 12v motors is
0.00?
Actually, since motor heating is proportional to I squared, and motor heat dissipation is proportional to the surface area of the motor, plotting these two for motors with surface areas larger and smaller than your test motor might give you a straight line on linear-linear paper. Then you read off the predicted I squared value for your motor and take the square root.