i have found bigger the better....

i bought a OiOi messanger nappy bag to start with and got to the point where i couldnt fit everything in it! admittedly i do carry a lot of just incase stuff but even if i took the bare minimum it was still a tight squeeze....

my DH bought me a Isoki Hobo bag for mothers day and i LOVE LOVE LOVE IT!!!!!!

its not structured, as in u can shove stuff in and it just buldges more and more!
it comes with a change mat, a insert to keep ur nappies, wipes etc tidy and a food bag.
couldnt live with out it now... oh and its reversible too, so you can have a patterned outside or a plain one