How to change font size on the text buttons ?

Hi, Would you please provide to me the CSS code so that I can change the font size on the texts on the buttons ? For example, I would like to reduce the size of the text "ENVOYER" on this page from my website : https://bastienchilloux.com/contact/ I just wanna have a CSS code affecting all the font size text buttons, to be able to adjust it to my entire website font size logic. Thanks a lot in advance, Take care