Any size should work since the portfolio resizes them all to the same size. As long as they’re not too wide or too tall and thin they should all show up the exact same.
Also, you may want to give the Regenerate Thumbnails plugin a try – http://wordpress.org/plugins/regenerate-thumbnails/ This plugin will resize all of your images to match the size of the thumbnail specified in the theme. It will make them look a little better.
Do you have a link so I can see what you’re referring to?