Small DC motor Caps - what size to use?

Hi guys,

A couple weeks ago I was asking about the small caps on the DC motor I
was using (R/C car motor). We covered the whys, but not the values to
use.

I have another motor I will also be using. This one is from a 6 volt
cordless screwdriver that originally was intended to run on 4 AA
batteries. This motor does not have any caps on it and from what I
learned last time, it would make good sense to solder a couple on
while it is still accessible.

Only thing is, I don't have a clue what size caps to use. Is there
a "rule of thumb" or "this should work fine" range you can give me.

Thanks

Chris