I wanted the whimsy of EVERYTHING but the intelligent species, and their constructs, being HUGE. Trees are high-rises, wolves are horses, horses are bears. But, a few folks keep doing me the Syndrome meme, "but if everyone small, no one small". I didn't think this was that hard a concept to grok.
As to extra-planar entities: I stole a concept from the latest Magic the Gathering set, Bloomburrow: anything that enters the plane is transmogrified into their closest equivalent in the "small" size bracket (as defined by Fifth Edition). Elves are Forest Gnomes, Dwarves are Rock Gnomes (best match I could find. Dwarves are medium-sized, for the record), Humans are Halflings, Dragonborn/Lizardfolk are Kobolds, Orcs/Goblinoids are Goblins, etc. Planestouched become whatever their mortal half is, so typically Halflings. I'm still debating on how to do celestials/fiends. Why this is could be a major mystery for the setting.