You are playing World of Warcraft which is well known an old MMORPG game. You are in arena where you play against another player. You are a mage and the opponent is a rogue which can hide while moving until a spell hits on him. (so it is 1v1, Rogue VS Mage)
Rogue is a stubborn player and he just wants to take the game as long as possible, so your main task is to find him first!
The arena has a square shape (20x20 unit), and your AOE (area of effect) spell has a radius of X. You can cast your AOE spell every 5 seconds, no other spell has such a behaviour, and every second you and rogue can move at most 1 unit away from their current locations. Rogue can always see you and move accordingly not to be spotted.
What should be the minimum radius of your AOE spell to guarantee to find the rogue at the end?