Most doors are 30, 32, or 36 inches. When measuring take into account the width of screen doors, trim and hardware. Measure each opening through which the appliance will pass.
Today's dryers come with a wide variety of features any number of which may be helpful to you. However, they are only an asset if you'll actually use them. Don't get talked into buying a machine with lots of complicated features you'll never use. However be sure that you purchase a dryer with a sensor that will alert you when the clothes are dry. This allows you to reduce unnecessary energy consumption.
ENERGY STAR does not label clothes dryers because most of them use similar amounts of energy, which means there is little difference in energy use between models.