Personally if I'm viewing a gallery it's annoying when the pictures are all different sizes, particularly if they are too big for the screen or so small that they are not much bigger than a thumbnail. I would resize everything to be about the same size. If you have Photoshop you could easily do it with an action.
Not sure too sure how galleries are set up on web pages because I have not done one on my site yet, but I would think you could set the dimensions of the picture on the web page to a percentage so that it will open relative to the size of the users browser?