It makes sense. After all, Celestia is eternal and ageless (at least by any reasonable Pony time-scale). So making her body a standard candle for measurements has a certain logic to it. That said, I'd bet that Celestia likely somewhat disapproved, at least in private. She probably found a magical alternative although keeping the nomenclature of the Princess System of Units (sp units).



I would have thought that another unit of measure would be her wingspan and another the length of her horn.