Size and style is a matter of preference. I don't like stubby bar ends. I had a set of long bar ends and ended up cutting them off to about the length of the ones at the link. http://www.tredz.co.uk/.BBB-Classic-Bar-Ends_50569.htm That length gives me a comfortable wrist angle when I'm at the end of the bar ends and lets me place my thumb flat on the end cap when I want to do that.
I like narrow clamps(which don't require you to move your grips inward as much) like the ones on the bar ends at the link. Mine are like that(different brand though), and I've never had a problem with them slipping.