"Pick on Someone Your Own Size" is a common expression that warns bullies to choose opponents that are of equal strength or power. The statement implies that it's unfair to intimidate those who are weaker.
There are several synonyms for this phrase that convey the same message. One popular alternative is "fight fair" which suggests that aggression must be balanced, and both parties should have an equal chance to win. "Play by the rules" sends the same message, reminding people to refrain from using alternative tactics to gain an unfair advantage. Another similar expression is, "Don't kick a man when he is down" which warns aggressors against attacking those who are already struggling or vulnerable. Overall, these phrases underline the importance of treating people with respect, fairness, and kindness.