A full-size Queen's Square reflex hammer fits perfectly in the bag on the diagonal. I've had it since my junior year, and it's still looks brand-new. Also, it has a wide strap, which I highly recommend in any bag you look for, as it helps to distribute the weight a bit better.
I use a jansport beltbag---which SDN people tell me not to use in the US because only nurses wear fanny packs. But it's just sooooo practical! Hahaha 🙂