One of the worst revisions of any game system ever.
They took an elegant system and replaced it with something much less usable, much less elegant, and which addresses typical RPG problems to a much less degree. Instead of a single size quantifier, we now have two or three size quantifiers and we have to write stupid sentences like 'You can use a one-handed weapon in two hands' or else suddenly you can't grip the haft of your battle axe and lay a heavy stroke. Instead of any weapon be usable by someone in some capacity, every weapon is now usable only by someone in particular. So I as a DM have to start leaving small sized items everywhere if I have any small sized PC's, and any small sized items my small sized NPC's have effectively have 'protection from medium sized PC's' on them
Plus a surprising number of rules depended on the notion of weapon size and will now have to be explained in a new way. For instance, what is the definition of a 'light weapon' now that all weapons usuable by a character of a given size are one of two sizes. Is a dagger descriptively different than a longsword or are they both just 'one handed weapons'. Can I use a longsword in a grapple now? Can I use a longsword as my off hand weapon? If not, does that mean that the notion of 'light weapon' is still around and if so how does taking the explicit sizes away actually simplify the system?
I've never met a player, much less a DM, that didn't know that Gargantuan was the size class bigger than Huge, or than Tiny was two size classes smaller than Medium. Its laid out in a visually evocative way in the Player's Handbook for crying out loud. But if that was the only problem, then one could of simply number them. Fine is 'size 1', diminutive is 'size 2', etc. Or you could learn the nine simple and descriptive words which mean 1, 2, 3...
Sheesh.