Okay.
The Martial Arts Strikes skill allows you to deal Small Strike criticals with your unarmed strikes, and the Martial Arts Sweeps skill allows you to deal Small Sweeps criticals. Most natural attacks, like a lizardman's, allow you to deal Small Slash or Small Puncture with the Martial Arts Strikes skill. Monks deal Medium damage with their unarmed attacks.
So, does a Lizardman Monk deal Small or Medium Slash damage with his claws?