I think there are several standard sizes, probably based on standard sizes of a signature sheet (the sheet that gets folded and cut into multiples of 8, which is why PBs come in 24, 32, or 40 pages). This is such a hard question to answer at the dummy stage, as if you look at pubbed PBs there are SO many different sizes and page orientation. I think 8x8 and 9x9 are also fairly standard. If accepted it's going to change most likely anyway depending on house style. When I was creating dummies I just chose if it should square, landscape or portrait (what worked best for the story and illos) and then kept it somewhere within 9x11 or less (the size of my sketchbook!), most times basing it on the dimensions of a similar pubbed book. But it's been so long since I've been able to think about PBs I'd totally take with a grain of salt.