What does "poker size cards" mean?
Your customer is right.

Playing cards come in two sizes - poker size and bridge size.  Poker size is 63.5mm X 88.9mm. Bridge size is narrower at 56mm x 88.9mm. This makes more sense in old fashioned imperial measurements - both are 3.5 inches high, but poker cards are 2.5 inches wide while bridge cards are 2.25 inches wide.

What your customer is saying is that he knows he wants poker size, but when you give measurements he doesn't know if that is poker size, bridge size, or some other size.