An ellipse in Medusa is not a single curve, it is 4 curves. If you create an elipse and edit it so you can move around the points, you will see that there are 8 points to define it. The normal way to change the ellipse is to draw a group box around the three points at one end and move them by half the change required, then move the opposite end by the same amount in the opposite direction. This is all helped by creating some offset construction lines from the points of the ellipse. AFAIK the Medusa way of creating lots of construction lines and deleting them immediately after use is uncommon, but powerful once you get used to it.
In the full-fat Medusa you could drop into BaCIS-1 mode, and write your own macro to change an ellipse based on major+minor axes, but that runs at GBP4000 in the UK.
There is another possibility, to define the ellipse as a Parametric construction, where you could directly dimension the points, but depending on how much changing you want to do that may be overkill. However if you are going to need a lot of ellipses, create a parametric sheet with just an ellipse on it, change the dimensions, then copy the resulting ellipse into your working sheet.