At the moment we're unable to edit the size of the customer's billing address on the invoice in QuickBooks Online. We understand how important to be able to fully customise your sales form. I can add this to our report of Product feature/feedback and let our Product Developers know. If you needed to edit your sales forms, you can customise them by going to your Custom Form Styles. Here's how:
Click on the Settings icon.
Click on Custom form styles.
Click on Edit for Standard template design.
Click on Content or Design tab, to edit how your sales forms looks like.