If something is quite close by, it could be described as being a stone's throw away; even closer might be a hop, skip and a jump. I'm interested in these "units" of measurement based on human action. Do you know of other similar words, phrases or idioms that both describe an activity and act as a kind of measurement? They do not have to be limited to distance measurement.
In the southern US, walking distances are sometime described in looks. As in "Then you go 'bout three looks down the road and you'll come to...".
When you start down the road or path, you look for the most distance landmark you can see, and when you get there, you look again...
Obviously the nature of the path and terrain mean that the length of a look varies a lot.
I don't trust him any farther than I could throw him. (unit of trust)
There’s room in her bathroom to swing a cat. (unit of capacity)
I guess none of these are exactly units of measurement—you don’t say things like “It’s about seven stones’ throws away” or “How many hops, skips, and jumps is it to the store from here?” They’re more like thresholds that you measure something against.
In The Joy of Lex, Gyles Brandreth proposed a unit of measure, the millihelen, defined as the amount of beauty required to launch one ship.
Some old machinists are fond of the phrase "within a c*nt hair". (Sometimes the adjective "red" is added.)
Also "near as damn" (although personally I'm uncertain just how near that is...)
And a difference that "doesn't amount to a piss-hole in the snow"
First time through I forgot this favorite of my grandmother's "If it hadda been a sarpent it woulda bit ye" - used when your keys were on the table next to you all the time you were wondering what became of them, or your glasses were really on top of your head...
Don't forget the Smoot.
I just want to add a kind of overview and some links that might be relevant to the subject (in addition to many fine answers).
You should note that in traditional systems of measurement the units were often based on dimensions of human body. According to wikipedia these were often specifically based on proportions given by Marcus Vitruvius Pollio:
Leonardo da Vinci: Vitruvian Man
There is an article that list anthropic units such as the yard, the span, the cubit, the Flemish ell, the English ell, the French ell, the fathom, the hand, and the foot.
If you read up on the history of these terms and how they become used, you will find that the all described some sort of human activity and that was the reason these were accepted as units of measurement.
NOTE: I found stone-throw as "steinkast – stone's throw, perhaps 25 favner, used to this day as a very approximate measure of a short distance." under Norwegian units of measure.
They list an interesting unit for medium distances - rast which is lit. rest - a distance you would travel without rest (estimated at approx 9 km).
X bowshot(s) away comes to mind. Lord Bern uses it in C. S. Lewis' The Voyage of the Dawn Treader:
"And about five bowshots hence, when you get open sea on your port bow, run up a few signals."
Often used metaphorically as well as for actual linear distance, just a (short) step away seems to be getting more common over recent decades.