Heat shrink tubing has different shrink ratios, like 2:1, 3:1, 4:1 and 6:1. These describe the maximum size change before and after the shrink process. For example, the size of the shrink tubing is reduced from 10mm to 5mm, if the shrink ratio is 2:1.

So firstly, you need to measure the diameter of the biggest part and the smallest part of your object to cover, for example, 11mm and 5mm, then you will choose a tube of 3:1 shrink ratio, 12.7mm size.