Exact range of XP gain?

Solution 1:

To answer your questions:

• The exact range is 1600 (source) :

Minion waves calculations assumes a Summoner's Rift lane, no increased experience sources, no neutral monsters are slain and one champion is in 1600 range of every opposing minion death in said lane.

• The range remains the same but different units have different hitboxes.

• No there is no way to see an indicator for 1600 range (there is no champion with an ability with exactly that range. Just picture 16 teemos next to eachother (or a bit less than a rumble ult)

• The experience radius of the minion doesn't change but since your hitbox is larger you can stand a bit further out. (only the outer limit of your hitbox is needed for the experience). This is because the experience range is around the minion, not the champion and counts as a self targeted aura.