I am playing Zac in League of Legends and I noticed that his size changes throughout the game, but I can not find a description of him changing sizes anywhere in his abilities or passive. Is there anywhere that explains how this happens and if it affects anything?
This is explained only in Zac's wiki page, and nowhere in-game or the client. I find this really annoying because this is kind of important (or at least interesting) stuff to know if you're playing Zac.
Zac has an innate passive that has two effects. First:
INNATE - THE SECRET WEAPON: Zac gains increased percentage size equal to 1.5% of his bonus health, capped at 35% increased size at 2333.33 bonus health.
Thus, as you gain bonus health via items you buy throughout the game, Zac will get larger. This is capped at 2333.33 bonus health, so if you get more bonus health than that, you will not grow any larger.
The second part of this innate ability reads:
Zac's size also reduces based on his missing health, down to 70% total size while at 0% health.
Which means that as you either take damage or damage yourself by using Zac's abilities throughout the game, Zac will get smaller (and his voice will get higher pitched). You'll notice that as you get healed or take damage, Zac's size will continue to vary, as his size updates in real-time as his health pool changes.
Like all other champions, when Zac increases his size, his hitbox radius is larger, and he can be targeted by edge-to-edge targeted abilities and ranged auto-attacks from further away than the center of his hitbox (i.e. he's bigger, you can hit him from further away). When he's at his lowest possible size value when he's at 0% health, his size is 56 units, which is 1 unit larger than very small champions like Annie and Amumu. These size changes do not affect the range of his abilities.