The easy way is to give them a new unitstat inside their milestone instead. This unitstat decides the unit's visual size in tactical combat only.

UnitStat_TacticalScaleMultiplier

It defaults to 1. It is used by spells such as Grow and Shrink, and will thus affect how those spells change the unit's visual appearance.

The hard way is to provide them with a new "cutscene". The cutscene of each unittype effectively decides where the camera is in relation to the unit for special display screens such as unit details and when designing units (imagine the camera and the unit alone in a box). You can see the cutscene in each unittype that is linked to those in data/Cutscenes/.