Sizing of laptop messenger? Custom vs. Mass-produced?

I'm the proud owner of a first-gen laptop messenger, from March 2005. I can't find its dimensions anymore, but it's clearly neither medium nor small. I've just gotten a new Lenovo Thinkpad T61 with the 15.4" screen that won't fit the old one, so I want a new messenger. I'm concerned about the sizing. I almost bought a Large size, but don't want it to eat me; at the same time, I really want to make sure it fits my laptop. The informational pages swear that it'll fit (being 14.1" wide-ish), but there are quite a few complaints on this forum. Would my best bet be a medium Classic Messenger plus a supplemental sleeve? I really don't want the Commute or other dedicated Laptoppy bags.

I've also heard that the mass-produced ones from third-party vendors are sized slightly different. Any truth to these rumors?

Thanks for helping! I'm really excited to start carrying a Timbuk2 again!