I don't know the dimensions of the brushes, but they are large enough to contact two commutator bars as the armature turns- one bar is "falling off' so the speak and the next one is "rising" in the magnetic field from the field coils. Without brushes to look at, memory says they are slightly narrower (or shorter) than the length of the commutator bars by about an eighth of an inch. They are about a quarter inch thick, and probably less than an inch in height- maybe closer to a half inch in height. Starter brushes are typically bronze rather than carbon, and the commutator should not be under-cut- that is the mica between bars should not be cut below the commutator circumference as is done with a generator commutator with carbon brushes.
Hope this helps.
Harvester
