I used a Trijicon RX01 with a 6 minute dot.
That dot size was larger than I would like (i.e., large enough to be nearly impractical to me).
My last optic was an Aimpoint Micro with the 2 minute dot, and that was an excellent dot size. At this point, I really wouldn't consider putting a red dot on a carbine that was larger than 4 minutes, for sure.