Is there a way to change a layers symbol size from paper units(mm) to map units(m) once you hit a certain scale without adding duplicates of each layer? QGIS 12.2
You could also set the symbol size in map units, then press the "spanner" icon which appears next to the size. This allows you to set either a minimum and maximum size in mm for the symbol, or a scale range to limit the symbol scaling to.
Rule-based labelling can do it. Define two rules with disjoined scale intervals and set the other parameters for each rule.