I'd like to second this feature request. The 'maximum' should be the maximum width or height (which ever is greater). The other dimension should then be reduced proportionately so that the final image remains in the correct ratio.
Currently, there is this functionality ... somewhat. If you uncheck "Use Cropped Square Thumbnails:" in the Admin -> Options tab, it will set the maximum height to the maximum size you set. I'm hoping to get on this feature soon after the next release, in addition with cropping and rotation options. I can maybe come up with a hack for you to try in the meantime though. I'll have to take a look and get back to you.