Any bigger poses a safety risk. Their little heads can get stuck. Cockatiels need light, but don't put the cage in direct sunlight. If no light is available from natural lighting, supply a full-spectrum artificial light. This lighting is necessary for the bird to utilize vitamin D. Make sure the cage is in a well-ventilated area.
Fumes are especially hazardous to birds.
One serious fume that has caused many deaths is Teflon coated pans. They emit a lethal gas when over heated. Since it's not possible to know exactly where the overheating point is. I have never, nor would I ever use Teflon coated pans around my birds. Another product that is Teflon coated is some varieties of heat lamps.