Brussels sprouts can very a lot in size so make sure to adjust your cooking times. Also, try to select all the same sizes for consistent cooking.
Preheat oven to roast at 400°F. Lay foil or parchment on baking sheet. Set aside.
Transfer brussels sprouts on prepared sheet pan. Bake in oven for about 20 minutes, then gently toss and mix the brussels sprouts.
Continue baking for another 20 minutes, or until the brussels sprouts are tender.