Jurgen,
I generally lurk on the list, but I'll speak up now since I have basically done what you are talking about. I integrated MPL (TkAgg) into an existing application that resizes (PyRAF).
The path I chose isn't particularly pretty, but it does the job and sounds like it might be flexible enough for what you are doing. I created an adapter class which wraps the MPL calls. The adapter is what the existing application calls to make plots instead of calling MPL directly. The adapter also keeps a normalized (0-1) copy of all plot data arrays (and any lines and other items). When the canvas is resized, I go through all my lines (Line2D), patches (Rectangle), and text, and scale each based on the new canvas dimensions. Then the new items are inserted directly into the Figure object, replacing the old values. This resizes everything so that it is correct when redrawn. I was initially concerned about performance, but the resizings are very smooth (thanks matplotlib!). You could also enforce an aspect ratio this way.
I had to write this adapter not because of the resizing issue, but due to the constraints of the existing application with which I was working. It did however make resizing easy. As long as your plotted items are relatively simple (lines, patches, text), this works very well. Using more complicated items like legends would not work as well unless you either allowed them to stay a static size or manually composed them of simpler objects. In general, this idea is probably more work than John's DPI suggestions.
Chris