How do I change the minimum required address information?

Within the Site Settings page, you'll find the Address Requirements setting near the bottom of the page. 

This setting controls the minimum number of fields required for billing address from your customers at checkout. Please make sure this matches the address requirements for your gateway's address validation requirements. Most gateways require Street Address and Postal Code, while Paypal requires a full address, and Bambora (formerly Beanstream) asks for full address and phone number.

Recurly recommends collecting postal code at a minimum.