High Torque Brushless DC Motors in Small Package
What max diameter can it be? 

kg.m = oz.in/1385.   

Power ~= RPM x kg.m
= 100 x 200/1385 = 14 watts.

That's inside the range of battery electric drill motor but the 8mm length is "very difficult" depending on what diameter you can tolerate.
IF diameter is "free" you can do it by making a large dia wheel. A Fisher &amp; Paykel smart drive motor probably meets your requirement. They use Ferrite magnets. Dia will decrease substantially with rare earth magnets.

Drive Calculator program old excel and new standalone pgm.



Excellent - relatively simple Simple analysis for Brushless DC motors

Drive Calculator V3.4 free

Old Excel version and link to new

Manual for V3.4

25 e-flight calculators - some relevant

Motocalc download - 30 day free trial then $40

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