The author expressed his "disinterest" in adding a max-height feature:
[...] the purpose of this plugin is to resize in width only, to avoid breaking the layout of the used theme. If you want to resize the height to a given width you can change the source code or use another plugin where you can resize things. [...]
so I added it myself. It was just a small hack, all credits stay with Anton.
Download here: http://www.mores.cc/better-resize-at-upload