Looking over some on other sites
it seems the smallest i can find accross motors is 6.0mm
Nova makes
5.4, and 6 for there smallest some buggy motors come with 5.4 not sure what color it is.
6.0mm is white
other nova and picco based motors 6.00 is as small as I can find for them. of coarse the other nova motors a 6 or 5.4 would fit, as for any other brand i have no idea.
Food for thought anyways.
let me know what others find