PS I think Smogon and its evasion clause suck.
And that's your opinion. I personally don't like being walled by a Minimizing Muk in storyline; the same strategy can be produced in Wi-Fi or friendly battles. And I know this because it's happened to me -- trainers (who ignore Smogon's clauses) fall upon their last Pokemon who uses similar said strategy and will drag the battle on for an indefinite unnecessary amount of time.
And the evasion boost stat increase paired with recovery moves is just an example. That doesn't include other moves that increase evasion or accuracy. The clause does make certain Poke "useless" due to certain abilities (Keen Eye, for example), but it also brings another edge to making teams that don't pertain to Smogon's clauses, therefore creating more "replay" value. Smogon is designed with speed in mind (along with other things) and boosting said stats when combined with other technical tactics can
be an intellectual form of defensive combat (possibly offensive as well) but is often too slow (annoying) and (through certain perspectives) "cheap" when playing in a game that is intended for or preferred to be fast-paced.
But this thread isn't intended to argue that point...
And as for a Poke's size determining it's said "visible" evasion stat, the game is and has somewhat always been correlating this idea with stats like Speed and Def, along with a movepool that would increase certain stats depending on the inspiration behind said poke.
But if the evasion "stat" was to ever be visible, I imagine the same idea would be applied to it.