Hey, I'm looking to buy a Timbuk2 classic messenger bag. I also want the snoop camera insert because I have a dslr a one or two lenses that I carry around sometimes. I currently have a 2009 13 inch MacBook pro but I will be upgrading to the most recent MBA. What size of classic messenger should I get for the MBA and snoop camera insert? I think the small works, but does the extra small work too? Thanks.