I didn’t find the solution, but I managed to work around:
I changed the default width of the buttons in the CSS template (unfortunately, this applies then to all the buttons).
This is how I did it:
Select “assets” on the menu on the left, then theme.css on the top of the page. Search for “button” in the css-text. There, under “padding”, change the second of the pixel elements , who stands for the width.