Setting Button Module Widths using Custom CSS
One way to set the widths of your button module buttons is using the custom CSS options Divi provides in the module settings. This can be used to set the minimum width on the buttons individually. You can do this by going into to "Advanced" tab of each button module and pasting the following into the "Main Element" box:
Setting a Site-Wide Minimum Button Width
Divi Booster contains an option for setting the minimum width on buttons within Divi. You'll find it at:
Divi > Divi Booster > Divi Builder > Standard Builder > Set minimum CTA button width
It sets a minimum width for all buttons on the site, so it's definitely worth considering the impact on all other buttons on your site – these would also be increased in size if smaller than the minimum width set in the option. Note that this option affects not just the button module, but other modules which display buttons such as the CTA and slider modules.