Midorima was banking on him knowing what it was or else it wouldn't have landed as a threat. More than that, he would've had to explain it and he definitely didn't want to do that.

"You wouldn't be able to walk into the store to get anything worse than that if you can't stop laughing at this one."

Midorima says it more under his breath than directly to Takao, but it's likely still audible. Midorima is focusing now on picking out a shirt or two for himself on a rack nearby with his size.

"Are you going to try them on?"

Midorima knows Takao's size well enough, but he wouldn't blame him if he did.