Badly need to set the Bike Speed Sensor wheel size higher than the Vivoactive4's max of 9999 mm

How did you compute 12 meters per revolution?

By comparing the speed and distance reported by the bike to the speed and distance reported by the watch at different wheel size settings, and then just inverting the math (aka algebra, lol). It's extremely consistent, so it should work if I can just enter a bigger size.

Currently I'm going back in after the workout and editing those stats manually, which isn't quite the same and misses out on a few features.

But at 12 meters per revolution, the sensor must be on a very slow moving wheel? Even if it was on the crank, you would not even be close to 10 meters. Or is the indoor bike reporting crazy speeds like 70 km/h?

Ahh, because the sensor is not on the wheel, because that isn't possible. It's on the crank, which spins at a fraction of the rpm of the wheel, hence why I need the "wheel size" to be a much larger number than would seem realistic for an actual wheel....to account for the gear ratio. So it's just a math problem, not an actual 12-meter circumference.

My actual normal speed is 18-22 mph (30-35 kph).

If you pedal at 90 rpm at 35 kph, the wheel size should be 6481 mm.

I'm sure your math is probably right, but I'm not pedaling 90 rpm. I average about 50 rpm, which comes out to around 31 km/h or so (that's just a guess from memory, not an estimate). There's a pretty big gear ratio between the crank and wheel.

Here this is more accurate, from my last two workouts:

21.1 mph (34.0 km/h)
51 average rpm

20.4 mph (32.8 km/hr)
50 average rpm

Unfortunately the rpm rounds off, so I don't have better precision. So I have to take the stats from a few workouts to average them out.

But when I set the wheel size to 9999 mm on the ant+ speed sensor, the bike's distance is always about 20% higher than the watch is reporting.

51 rpm is an extremely low cadence. I doubt that you would be able to go 34 km/h average speed at that cadence with a real bicycle outdoors. This indicates that your indoor bike is reporting an unrealistic speed/distance. I think you are better off with setting the wheel diameter at a value below 9999. 

Normal cadence for performance bicycling (speeds over 30 km/h) is around 90. You indoor bicycle would be reporting 60 km/h in that case. Who can go that fast with a bicycle? The current world record for one hour is 55.

In theory 51 rpm on an assault bike is 186 Watts. Of course that is not a real bike either.