I recommend the Vortex caps. Their sizes are labeled clearly on the front of the package and they fit perfectly. MUCH better than Butler Creek. They flex well, fit SNUG (won't twist once installed), they lock closed very well and the objective cap opens in 2 positions (up & back at 45 degrees or lock backward horizontally flat to the top of the scope). I was going to get Butler Creek caps too because I kinda like that push button on the ocular lens, but the Vortex caps are soooo much better I didn't hesitate to get theirs. Yeah they're twice the price, but the quality has value. Also Vortex makes it EASY to size & buy the caps. Butler Creek has like 50 different sizes, it's insane.
Butler Creek has a size chart
on this page and Vortex lists the objective outer diameter
on their website as 2.55".