For best results, use beets that are similar in size so that they take the same amount of time to roast. If you have beets of varying sizes, you can cut larger ones in half to make them closer in size to smaller beets. Alternatively, you can group small beets together in one foil packet (or two if needed) so that they are more similar in size to large beets. Finally, you can keep your beets as they are and just remove the foil packets with smaller beets from the oven earlier than you remove the larger beets.