Be careful and never go faster than you feel comfortable or faster than the conditions allow. Many factors come into play. Visibility, road surface, run out, wind, fatigue. etc. Naturally a bike with larger wheels feels more predictable at high speeds going downhill.

The fastest safe speed is very personal. But, I think, generally an individual will feel more comfortable going downhill at higher speeds with larger wheels, but it's been a long time since I ridden a large wheeled bike.

Edit: Second thoughts