It's maybe worth trying to translate some of Quake's basic units into real world ones to get an idea of how player abilities compare to those of a regular human. Not that anyone's hoping for any great realism here: any human that can walk away from a shotgun blast to the face is clearly a little above average in some respects. And we'll take fun over realism any day. But it is interesting to get an idea of just how fast and powerful the Quake-guy is.
The first thing to note is that Quake's vertical and horizontal axes seem to work on different scales. Quake-guy doesn't seem exceptionally tall (he's quite a stocky looking fellow) and stands 64 pixels tall. So along the vertical axis, one pixel per inch seems about right. But in the horizontal plane, pixels seem to be rather smaller in general. It seems reasonable to reckon that in this direction there are about 50 pixels to a metre, or approximately 15 to a foot. Apart from anything else, the round figures make the sums easier.
Velocities (or speeds) in Quake tend to be measured in pixels per second. The maximum speed that a player is supposed to be able to run at is 320 pixels per second. That comes out to around 23 km per hour, which isn't so unreasonable. He jumps 45 pixels up in the air - almost a metre. That's a little more impressive. But then again the poor guy can't use use hands to climb things, so I think we can forgive him having a spring in his step. Lengthways, he can jump the best part of 250 pixels (5 metres), and that's on top of his remarkable ability to stand beyond the edge of a ledge (made possible by his rather fat 32 pixel bounding box.) But gravity in Quake is much lower than it is in real life too, as you have probably noticed!