Help setting parameters

my code:

distancing_mc.onEnterFrame = function() {

xdist = Math.round(_root._xmouse - _x);
ydist = Math.round(_root._ymouse - _y);
distancefromthis = Math.round(Math.sqrt((xdistxdist) + (ydistydist)));
circle_mc._width = (100-distancefromthis)+20;
circle_mc._height = (100-distancefromthis)+20;

I have a circle that gets larger/smaller depending upon where the mouse is in comparison to the circle. How would I set parameters so that the circle would shrink only to a certain size and also grow only to a certain size?