First of all. you have to ask what you need to take with you. I'f you're with Air Atlanta and have to carry your own Jepps around with you, then buy a big one. But some airlines don't require you to carry anything. My last one actually banned you using anything but compny literature which was all on the flight deck anyway. One guy I used to fly with turned up with his license in one shirt pocket and his passport in the other - and no bag.
Me, I carry my licence, passport, my hat, an A4 folder with my roster and telephone numbers, a couple of pens and the torch required by the local operating authority - and that's about it. I use a Samsonite shoulder bag 'borrowed' from Mrs Winterland, it's lasted about 4 years now and will do me for some time more.