Well they are designed to be a column layout, so in the theme options you can switch them to 5 columns which will make theme not fill the whole space since you have three.
to change the font size start here:
.home-iconmenu a h4 {
font-size:12px;
}
.home-iconmenu a i {
font-size:18px;
}
And you can adjust height padding with this:
.home-iconmenu a {
padding: 0px 20px;
}
But adjusting the width to be a size and not a percentage is not really the intent and if you are going to go that far I would just code your own buttons.
Kadence Themes