I would avoid baking subjective measures into the stadard. I do understand that people want the simplest possible solution, but I'd stick to objective measures. Subjective definitions will change over time, causing bad user experience, e.g. "why are all applications too big for my 7 year old phone?" because the "medium" tablet is now better served by the "small" category, or "half of the applications don't fit" when someone makes a small phone which is 10cm in height but the software center accepts both "xsmall" and "small" because neither is really defined. If anything, I would rather disseminate templates for those sizes, made up of objective measures, for copy and pasting by packagers.