Size the Bag to the Machine, Not to the Label
Bag sizes are quoted as screen diagonals, and those diagonals were fixed when laptops were 16:9 and much wider than anything Apple builds now. A sleeve sold as 15-inch was cut for a chassis noticeably larger than a 15.3-inch Air, and a 14-inch Pro is wider than a 13.6-inch Air despite what the two numbers suggest. Compare internal dimensions instead.
Everything else follows from that fit: padding placed where the machine will actually be knocked, a compartment that keeps it clear of the floor of the bag, and a separate pocket so an adapter never sits pressed against the lid. Sleeves, messengers and backpacks each handle those three things differently.