The option in ACP attachments lets you set the maximum width and height for thumbnails but only together, not separately. Depending on how your page and pictures are laid out, this causes tetris-like thumbnails. It looks inconsistent.
A suggestion to make it look even is to allow admins to set the max w/h separately so that I can make all thumbnails the same height. Setting a high max width would let any extremely wide thumbnails not get out of hand.