I found one more Washington city that allows printer sized petitions, Bellingham (from Bellingham Municipal Code 1.02.030):
D. Petitions shall be printed or typed on single sheets of white paper of good quality, not less than 11 inches in width and not less than 8-1/2 inches in length with a margin of 1 inch at the top, bottom, and sides of the petition for office use only; and each sheet of petition paper having a space thereon for signatures shall contain the text or prayer of the petition. For any particular petition, all sheets of paper shall be the same size.
I found that reference while following up on an initiative being filed up there to protect Bellingham Bay. I'm sending the same email that I sent to all the other Seattle initiatives about whether they're putting a petition online.