Depends on the event and what you're trying to sell at the event. At something like a Kid's party, everyone will know what you're doing, and they're not likely to want a large shot. At a kid's football game, people might not know you're there, and if they did, they might want a print large enough to hang.

People are pre-conditioned to think of 4x6" prints as being cheap. The larger print sizes of the Pixma would add some value, and potentially some interest. If people see someone walking away with an 8x10 or 13x19 print, they might notice and come have a look at what you're doing for themselves.

This is all theoretical... I've never showed up to an event with a printer. I picked up a Pixma Pro-100 just 2 days ago, the thought of doing this had already crossed my mind. People are there already, and may be encouraged when they see someone else buy a print.