8.5:1 gear reduction. Shaft diameters are measured at the collar.
3/4 Cello There isn’t a size designated by Wittner for 3/4 size cello. However, the head size of the 4/4-7/8 is very much within range for a 3/4 cello. The 1/2-1/4 would probably look a bit small.
Please, use the diameter chart.