What does "for size" mean?
The full expression is "try (something) on for size." This is an idiomatic expression meaning to tentatively test out or try something to see if it is suitable or good. It originates from the act of trying on clothes to see if you like them. Here, the speaker has made a pun. He is antagonizing Gru by asking him if he likes his laser gun attack ("try this on for size") and also making a pun, as the attack's effect is to shrink things (relating to "size"). Ex: Are you tired of regular old coffee? Try this on for size! It's called Dalgona coffee and is the latest coffee trend! Ex: The probation period allows new employees to try positions on for size.