I am leary of ordering 'one size' items but these fit very nicely. I have fairly large arms (c.14" biceps) and these fit comfortably - no pinching or slipping. Though I just recieved them and haven't used them on a ride it seems unlikely that they will slip due to the design of the guards. They also hold the elbow joint in an 'open pack' position to keep it less likely to be injured in a crash. (Open Pack = slightly bent, jointsurfaces fairly fair apart, tendons slack). There is some gapping between the forearm section and the arm, which I'm sure will go away as I sweat them in.
I'm giving them a 4/5 only because I haven't had a chance to ride in them yet.