A 12" signal is the only legnth that meets the RIDOT traffic safty standards here. The USDOT allows both 8" and 12" diameter lenses though.

Standard MUTCD sizes are 8", 9" 12" and 16".

8 and 12 are vehicle signals, 9, 12, and 16 are pedestrian signals.

Here 12 and 8 inch signals are allowed, 12 inch for pedestrian signals.

My state only allows 12" signals for vehicles and 12" for peds.

8, 12 vehicle, and 12 ped were used here. But the new standard is 12 vehicle and 16 ped.

To add on to what Niall said, the 8-8-8s are still allowed as a secondary signal head on smaller roads or for bus/transit signals, but the 12-8-8 is the most commonly used signal nowadays though.

The 12-12-12 is usually only seen when the green section is a arrow, on high speed roads and on left turn signals.