I don't know if such a software exists but what you would need is
- a drawing tool giving you the (approximate) length of the curve you draw (made of ellipses, lines, splines,...) ; it may exist
- a scaling tool, so that you can rescale your modified curve to make its length equal to what you want ; this surely exists.
As for the case of the circle, your sketch is not correct. In fact, there is no simple formula to find the length of an ellipse (what you call an "oval"), but you can compute it approximately by using the formulas given in the wikipedia.
A simpler task (in this situation) is to make the area constant. Indeed, the area of an ellipse with semi-major and -minor axes and is : to keep the area constant, you must keep the product constant. In your case, for instance (in mm), so that an ellipse with semiminor axis 2.5mm and semimajor axis 10mm would do fine. But not for the circumference.
I hope this helps you.