To change newsletter signup button size, you can use this code to Custom CSS box.
button.newsletter-form-button.sqs-system-button {
padding-top: 10px !important;
padding-bottom: 10px !important;
padding-left: 40px !important;
padding-right: 40px !important;
}
span.newsletter-form-button-label {
font-size: 16px !important;
}
