I am unsure of how this would be done code-wise, but as far as the FRED interface is concerned, as there are typically 3 sizes of asteroids: starting size, shoot it once=medium, shoot medium=small..., I see sliders similar to the RBG brightness for the background editor. By default, the large size would have its slider all the way over at 100%, while medium and small are at zero. As you increase or decrease one, the others adjust so that the sum total of all asteroids is always 100%. The problem with this simple interface follows if someone were to create (via tables) an asteroid field with more stages of breakup and more interim sizes of asteroids. A drop-down menu could be used to select which asteroid you want to adjust the percentage of.
While we're on the subject of asteroid field upgrades, I also submit that the field type which currently uses a one or the other in terms if rocks or debris, passive or active me made to not be mutually exclusive. Replace the selector with a drop-down menu and allow many types to be included in the same field. For example, a convoy is passing through an active field when they encounter a debris field left from a previous convoy.
This also brings up the subject of not just many types within the same field, but of multiple fields within the same mission (an old request).