Pan sizes. Recipes – mine and others – will generally tell you “use a 9” pan”, or “use a 13” x 8” dish, or whatever. If you have those, great. If you don’t, don’t stress and for Pete’s sake don’t go out and buy it just for that. In fact, in general, don’t go buy stuff because ONE recipe calls for it – you’ll never use it again and it just ends up taking up space.
Instead, use something that’s close to what the recipe calls for. If you have an 8” pan and not a 9”, that’s fine.
Just tweak the baking time a little. For instance, in an 8” pan the batter is a little thicker, so you might have to cook it a little bit longer. Just keep an eye on it (which you should be doing anyway) and you’ll be fine.