In the Customizer section, you can adjust the form field width by choosing the options from the field-width drop-down.
Follow the steps bellow:
Step 1: Click on the 'Edit' button to open the Customizer
Step 2: 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 from the drop-down
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: