Thread:Tyranitar12/@comment-26252250-20170223192613/@comment-26252250-20170223205710

ah kk, surrender at 20 had the latest ones at 300x300

The largest size for an icon I've seen (whether ability, summoner, or mastery) is 512x512. I assume this is the size Riot creates them at. Ideally, every single icon would be uploaded in their highest resolution and with no compression/pixelating.

Which is why I always recommend digging through release/reveal/sale sites for any one you might find.