Since you've used "em"s to set the dimension of buttons, you can increase/decrease font size of the button according to the viewport or you can use media queries to modify its dimensions.
Also, I would recommend setting font size of the button using "rem" instead of "em", because "em" will set the font size of the element with respect to the font of its parent element. If you set the proper font size of the button, then you could adjust its dimensions using "em" for padding or width, and modify them by changing the font size or using media queries.