Karthus: Confirmed Short
A buddy of mine on the Boards asked a Rioter how tall Karthus was and got this response:
https://cdn.discordapp.com/attachments/451454411084922892/605512187829551202/unknown.png
If the image isn't working or it's too small, the relevant text says "the information given back in the day about him being 1.60 should still hold true."
That's 160 cm, or 5'2".
Karthus is 5 foot freaking TWO without his hat. Yes, his color story describes him as "towering and broad-shouldered," but he floats like a foot off the ground and wears a really tall hat and pauldrons, so that probably accounts for that.
And it makes sense. Dude spent his prime growing years on a rats-and-rainwater diet, that can not give a growing boy the nutrients he needs to get tall.
"But Notarobot1006," you might ask, "what about Sylas? How'd he get so big eating rats?" Well, those were Demacian rats, which eat Demacian food from the fertile Demacian soil and are thus jam-packed with protein. Also Sylas probably got at least semi-regular meals on top of that.
Now that's out of the way, here's a really fun height comparison chart a different buddy of mine made with Morde (13 feet/396 cm), Karthus, and Nightfall:
https://cdn.discordapp.com/attachments/451454411084922892/605519417803014174/unknown.png
Thank goodness he can levitate or he'd have to carry on a conversation with Morde's shins if they met.