Might as well opt for blocks of 16x16.
[I]Video compression uses blocks of 16x16. If the resolution isn’t mod16, it will be padded (internally) to the next mod16 resolution and cropped back at playback time.
Hence you pay bits for parts of the image you’ll never see. So in CRF mode you have to expect a bigger file, in 2-Pass mode you have to expect worse quality at a given bitrate.[/I]
For my Xvid conversions if the source is 720 wide I keep it there.