2 small yellow onions can be subbed for 1 large; peel and cut smaller onions into quarters. Same goes for potatoes: if your potatoes are larger, quarter, or cut them into medium chunks.
If you have time, I highly recommend salting the chicken ahead of time (anywhere from 1-3 days), refrigerating it uncovered during this period. I like to do this on a quarter-sheet pan with a wire or
silicone rack set under the chicken for better air circulation. This technique,
inspired by Judy Rogers' Zuni Café Cookbook is a dry brine, and will set you up for a super-flavorful, succulent chicken with crisp skin (storing it uncovered helps reduce moisture which allows for better browning).