A 13" MacBook with BookBook will fit inside a bag made for a 15" MacBook and a 15" MacBook with BookBook will fit inside a 17" bag. The bag should be the next size up from your MacBook.
COVID-19 Virus Update: Twelve South remains open and appreciates your continued support and business immensely. However, due to our team working remotely, the only way to reach us is via email. We monitor email closely and will typically reply the same day during normal business hours (see below). Please reach out with your order number, concern or question at firstname.lastname@example.org.