A common term for surfers in measuring wave height is "overhead". Basically, a wave is measured in relation to the size of the surfer so for me overhead is 5'9", two foot overhead is 8'-ish, double overhead is around 11-12' and so on. Being at about novice level for surfing, anything close to overhead is really the limit of where I am comfortable even hang out in the water, let alone try to catch a wave. It's not just about dropping in then also for the getting out there and keeping calm while sitting and waiting for a wave- paddling into a wave that totally covers your field of view is pretty impressive the first time, even if it's only single overhead. That's a lot of water and a lot of force (though it was pretty cool to be somersaulted underwater when I was relaxed and had confidence that I could surface safely).
I think that kind of measurement is applicable to parkour. I like it because it is has an element of self-knowledge instead of being an absolute, and idea that I think is at the core of pk. Plus, as I discovered that first time looking at a 7' wave face, getting up and getting down are inherently linked (getting through a wave by duck diving is another example of finding an efficient path).
I think there's a fairly analagous relationship between skill level and amount of overhead one can handle between surfing and parkour (and maybe snowboarding, too?). Beginners may be advised to stay within their own height, intermediate maybe a couple feet overhead, advanced perhaps four foot overhead (10') to double overhead and beyond. Of course, handling heights is only one measure of skill level and not even the most indicative, but it's something to keep in mind when pushing limits on drops/leaps. Skill level to height is a little different when talking about wall runs and ascents, but going up sometimes means missing and having to maintain control on the way down, too.