> IME defining actual height/width in px is best left to the dev which typically means hard-coding. Its difficult for anyone to remember exact h/w across variations.

If it is going to be hardcoded in the componet, can't you just write
image.set("height", 200);
image.set("width", 300);

or 
image.set("maxWidth", 200);
image.set("maxHeight", 300);

and be done with it? The existing parbase servlet already handles resizing, either to a fixed size or with max/min. By default, these are stored in the component properties (for fixed size) and the design settings (for max/min), but if you're rolling your own component, you can set these to whatever values are appropriate.

I must be missing something...