They are very large. In fact, they take up so much of the owl's skull that there is no room for the muscles (such as human beings have) to move the eyes around. This is why an owl is able to swivel its head around to such a great extent (almost 180 degrees)
On average, owls can rotate their heads 270 degrees. If humans had eyes in proportion to an owls, our eyes would be the size of saucers. NF