Fork boots generally are pretty stretchy. I wrapped a piece of string around the slider and triple tree mounting areas, then stretched out the wire and measured the circumference of the mounts. Divide the circumference by 3 to get the approximate diameter. Sorry, but it's been so long ago I don't remember the measurements. Then I went googling and ordered a set of cheap universal boots that would fit. Put the boots on while installing All-Balls in the steering head. Cinched the boots on each end with UV-resistent zip-ties from Home Depot. All has been good for 4 years and 36,000 miles.