I use the Santini arm warmers with an external gripper at the top and they're brilliant.
Although I'm reasonably tall I have skinny arms and with other arm warmers I've tried they're either too short or too baggy (or both) so always slip down. The Santini ones stay put. The external gripper at the top grips against the inside of the jersey sleeve (as long as the sleeve is not too loose) which also helps, but mainly it's the stretchiness of the fabric that keeps them in position.
I'm a skinny 6'1" and use the medium size. They're plenty long enough and also snug.