Many distaffs throughout Canada were made by taking the upper tip of a spruce or fir tree, stripping it clean, and tying the tips of the branches together to form a cage over which the prepared flax could be loosely tied.
-Burnham and Burnham, Keep Me Warm One Night: Early Handweaving in Eastern CanadaI've never used a distaff. I rarely ever see them. I've seen a few wrist distaffs and the kind of distaff that looks like a pole.
One of the maddening things about getting your information from books is that an author can leave so much out. I have only the foggiest idea how much of the tree tip you use and what the final dimensions should be.
Going by one of the plates on the same page, and judging by the relative size of the given wheel diameter, I am going to guess that the bird cage distaff pictured is 20 cm from the base of the bent wood pieces to the tips where they cross. This does not include the rest of the upright (below the "cage") which fits into the spinning wheel.
The proportion between that 20 cm of height and the widest point of the ballooning bent wood pieces looks to be about phi, or the golden ratio of 1:1.618, the height being the longer measurement. I should say that the plate shows a rather refined looking wheel and distaff; the upright of the distaff is made of turned wood, not the tip of a tree trunk. I should also say that Burnham and Burnham do not call it a bird cage distaff, that is a name I have seen used elsewhere.