Messenger bags are really best purchased in person. Every manufacturer's idea of S/M/L/XL are a bit different. You can't really get a feel for craftsmanship from a photo. Some have water-tight storage areas, some don't. Some are cavernous holes for your stuff, some have compartments.

Keep in mind, cavernous holes are great for a working messenger who has to carry an array of package sizes. Not so great for a student with regular sized books, notebooks, and a laptop