Hi dental2019 ,
Welcome to the forums! 🥳
You can customize the width of the label for Rectangle styles by using this custom CSS:
.buttonizer.buttonizer-style-rectangle .buttonizer-button .buttonizer-label {
min-width: 100px !important;
max-width: 100px !important;
}
Add this into your WordPress' Additional CSS in Customize.
You'll need to change the value of both to your desired width.