Button size?

How do you measure the button? Do you just measure the plunger part or the ring around the plunger?
I read there’s 24mm and 30mm buttons.
The buttons on my Mad Catz Chun Li TE S stick measure 24mm across the plunger. Does this mean they’re 24mm buttons?
I thought 30mm was supposed to be the standard size?

30mm are the big ones on the face, 24mm on the small ones used for start select. You can tell visually by looking at them.

The button’s size refers to the hole size needed in the mounting surface for the button.

OK I see the start/select buttons are much smaller. Why don’t more people use the small ones for gameplay?
I have big hands (XL size gloves), but I have to stretch my fingers too much to reach all four buttons at the same time.
I could reach all of them easily if they were small and closely spaced.
I care because in Tekken you can use up to 8 buttons.

For Tekken, you only really need 4, 5 at most for tag games. Macros are designed primarily for pad play.

Because that’s what people have been using for 30 years. If you build/buy a custom stick or Hitbox you can use all the 24mm’s you want.