Your computer bag should be the next size up from your MacBook. So, a 13-inch BookBook will fit inside a bag made for a 15-inch MacBook, and a 16-inch MacBook with BookBook will fit inside a 17-inch bag.
External measurements of the BookBook:
13inch BookBook: 12.7-in x 9.30-in x 1-in (322mm x 237mm x 26mm)
16inch BookBook: 14.9-in x 10.4-in x 1-in (378mm x 264mm x 26mm)