OS X icons size

Question

What size should an application icon and menu bar icon for OS X be?

I can deal with small resolution displays but what about Retina - does an icon displayed on the men

Answer 1

adding on to Michael's answer apple are now requiring all the way up to 1024x1024px icons due to retina displays.

http://www.cultofmac.com/179738/apple-now-requires-high-res-1024x1024-icons-for-every-mac-os-x-app/