_marker setMarkerAutoScale[Local] true/false;
_bool = getMarkerAutoScale[Local] _marker;

There is a scripted solution, but it can get quite performance intensive if its 100+ markers adjusted each frame - would be way more efficient to be done in engine.

More flexible would be to have <SCALAR> as a factor with 0 being not scaled and negative inverse to ctrlMapScale. Probably too niche.