I knew there had to be something wrong here. From a not very deep look into our nation's initiative petitions, Washington State requires the largest — no less than 11×14 inches, almost twice the size of standard printer paper.
None of the states that I looked at in a search of the Initiative & Referendum Institute's research section require such a large petition. California requires exactly what I'm looking for, a minimum size of 8.5×11 and Florida requires petitions to be no larger than 8.5×11. Florida can only fit one signature per petition, though. One recent initiative turned in over 900,000 signatures. Can you imagine?
Anyway, here are the results. I didn't find a requirment for every state that has the initiative, by the way.
Arizona: 8.5×14 exactly
California: 8.5×11 minimum
Florida: 8.5×11 maximum
Michigan: 8.5×14 exactly
Maine 11×17 maximum
Mississippi: 8.5×11 exact
Missouri: 8.5×14 maximum
Montana: 8.5×14 maximum
Oklahoma: 8.5×14 exactly
Oregon: 8.5×11 exactly
Oregon (counties): 8.5×11 exactly
Utah: 8.5×11 exactly
Washington: 11×14 minimum
So, nine (updated, I realized now that Maine and Mississippi are different) out of the eleven states (and counties for Oregon) that I found information on would allow standard printer paper sized petitions.
What I can't figure out is why Washington requires petitions so much larger than the rest of the country. As I've demostrated, you can fit the text of a initiative on the back of a 8.5×11 piece of paper. I wonder how old the rule actually is and what the reasoning for it was.