I use the gallon size to pack anything that is 'spillable' - one bottle of shampoo spilled in a suitcase will teach a hard lesson! I also use a sandwich size in my fanny pack and keep the camera in it- I just zip it up when we are going on something that might get up a little wet- ie. Splash Mtn. Another tip I got from this board was to use the gallon size to pack DD7's clothes. I put her shorts, shirts, undies, and socks for each day in separate bags. There was not the 'what can I wear today?' problems. We also bring some snacks from home - trail mix, nuts, etc. for the parks- and use the snack size bags to put just enough for the day without carrying around too much.
I also pack lots of the different sizes. They don't take up much room and you will use at home what you don't need at WDW.