Standard SMT button size?
This is very typical button:



Name: Pushbutton Switches SPST OFF(ON) RND SMT MINI PB TACT SWITCH

It has very typical footprint available in many (if not all) programs for PCB design.

You can always look at stocks on Mouser or other supplier website to avoid buying "not typical" or "weird" product that may be not available later. Currently they have are over 10000pcs in stock.