This recipe makes about 16 cups (1 gallon), so you'll need a 6-quart slow cooker. For a smaller batch, cut the recipe in half and use a 4-quart slow cooker. A half batch heats faster in a 6-quart crockpot because the cider sits shallower, so check it a little early if that's the only size you have.
Use fresh, cloudy apple cider from the refrigerated produce section for the best flavor. Apple juice works in a pinch, but it's sweeter, so start with less brown sugar and add more to taste.
Ground cinnamon settles to the bottom of the crockpot, so stir the cider well before ladling each serving.