Hello,
Please add this code to the Theme Settings > Custom CSS > Global:
body.woocommerce-checkout input[type=text],
body.woocommerce-checkout input[type=email],
body.woocommerce-checkout input[type=password],
body.woocommerce-checkout input[type=search],
body.woocommerce-checkout input[type=number],
body.woocommerce-checkout input[type=url],
body.woocommerce-checkout input[type=tel],
body.woocommerce-checkout input[type=date],
body.woocommerce-checkout select,
body.woocommerce-checkout textarea {
padding: 0 15px;
max-width: 100%;
width: 100%;
height: 42px;
border: 2px solid rgba(129,129,129,.2);
border-radius: 0;
background-color: transparent;
box-shadow: none;
vertical-align: middle;
font-size: 14px;
transition: border-color .5s ease;
}
Please change the padding and height to increase the field, remove the lines you do not change.
Best Regards