For CNC router cutters, most sizes above 1/8” (3.175mm) cutting diameter will be on the same size shank as the cutting diameter. For example, a 4mm cutter would be on a 4mm shank. For sizes 1/8” (3.175mm) and under, most cutting diameters would be on a 1/8” (3.175mm) shank.

Notice that many router bits are available in both a 1/4" and 1/2" shank size. Whenever possible it's best to choose the larger shank size. The 1/2" bit shanks are substantially stronger and less prone to breakage than a smaller shank. In fact, a 1/2" shank has nearly four times the surface area of a 1/4" shank.

A down-cut bit will leave a clean edge on top but a ragged edge on the bottom; an up-cut bit will accomplish the opposite. This is great until you want to cut a dado with no tearout on the face. A down-cut bit will leave a clean top edge, but it sends the chips downward, into the dado where they have no place to go.

Shank Rockler Signmaking Carbide Bit - 3/16" D x 5/16" H x 1/4 | Cutting Edge for Lettering & Detailing.