In the Customizer section, you can adjust the form field width by choosing the options from the field-width drop-down.

Follow the steps below:

Step 1: Edit your Checkout page

Click on the 'Edit Customizer Template' button to start editing your Checkout page with Customizer.

Step 2: Edit the Checkout Form widget

Click on the Checkout Form widget on the left side panel.

Next, click on the 'Field Width' option.

Step 3: Select the desired field widths

Edit the field widths you want to change.

Customizer lets you visually see the changes that you make.

Take a look:

Step 4: Preview your changes

Click on the eye icon to preview your checkout page.

Here's how the checkout form will now look:

This is how you can adjust the form field width in the Customizer.