There are some that are 4.5 or 5 inches in size. these are the largest, since the smaller ones range around 3.4 inches. You have the ability to zoom with most of these products, so the screen size becomes less important with this aspect.
Even the 3.4-inch size is big enough to see all of the details in the room and on the infant. if the best baby monitors for twins were any larger, then they would not be able to be carried so easily in the hand.