Originally posted by pxpaulx Timbuk2's website has a laptop look-up, which sounds like you used it. If it said large, that is probably what will fit - they also publish interior/exterior dimensions which should make it easy to measure. I have a small messenger myself - it barely fits my dell/alienware m11x (11.6" screen) in the rear small pocket - our 15.6" HP would basically stretch out the entire space in the main compartment (though it would fit, with nothing else). Unless traveling, I use the bag just for whatever camera gear I want with me - without the m11x there is plenty of room to hold 2 bodies and 3-4 lenses depending on size - adding that laptop takes up a lot of room though, so keep that in mind and measure carefully!
Actually, their website is notoriously conservative about the laptop fits. Many people complain about their laptops being too small for the recommended bag. That being said, most of these people are using Macbooks, which are often differently proportioned. The medium bag lists its dimensions as being 19.3 inches long at the top (plenty big) but only I thought I saw somewhere that it might be only 14+ inches at the base (the photos do not look that way, though). They also don't give the interior dimensions for their bags.
My laptop is a Lenovo Z560 which is 14.8 inches long, 9.8 inches wide, and 1.6 inches thick. It seems like it would fit into a medium bag.
As for using a classic messenger bag, I think the classic bags and snoop bags are essentially the same except the inserts. I was looking at the classic because I already have an insert. I also assume that these bags have some give to them? I'm trying to avoid having to return a bag, and I don't really have any way of testing them out without making a 3 hour trip. Of course, I might if it really comes down to it (San Francisco itself).
I thought I had seen, searching these forums, that some people were using Timbuk2 messenger bags in general as camera bags for day outings, etc. and it seemed like a good idea.