The tool recommended in the first post will do it automatically, so you don't have to worry about that. If you're using something else and just want to know what the recommended settings are, then...
For a 4:3 aspect ratio, try this:
LanczosResize(660,440)
AddBorders(30,20,30,20)
For 16:9 aspect ratio, use this:
LanczosResize(660,330)
AddBorders(30,75,30,75)
I included the resize as well for obvious reasons.