I bought a size small, hoping that it would prevent them from sliding down.  They are blood pressure cuff tight when I put them on, but still no dice - they turn into forearm guards halfway down any remotely technical DH or enduro trail (as soon as it gets bumpy, basically). I spent the entire day pulling them back into place.

Lots of protection if you can get them to stay put but I am a normal sized guy and couldn't make these work for me.