As with any bag, it has the tendency to fill up with useless stuff until it's full or too heavy to carry.
In that respect, you will probably want the smallest bag that can hold:
- Kneeboard
- A minimal set of charts, flight guide, other paperwork
- Logbook
- Headset
- License & ID
- Ruler, protractor, flight computer, pen&pencil
- Fuel tester, some old rags to wipe stuff, dipstick
- Hi-viz jacket (sadly required all too often)
- Bottle of water and a packet of biscuits or something like that
- Sunglasses, spare glasses
- Possibly your GPS, Icom, PCAS, PLB (if you have any of those)
I've got the AFE soft pilot bag and it works just fine. I can even cram in a change of clothes if absolutely necessary.
(Just checked the AFE web site and that particular bag is no longer listed, but a picture is currently under the "Free Competition" header on the right hand side of the page.)