According to the National Weather Service, the average diameter of a snow roller is 10 to 12 inches (25 to 30 centimeters). Some are quite small — roughly the size of a golf ball. Extra-large ones can have a diameter of around 18 inches (46 centimeters).
You almost never see individual snow rollers. Either you'll see none at all, or you'll see a whole field dotted with them. That's because the conditions that create a snow roller do not affect one narrow strip of land. They affect large areas all at once. If one part of a field has loose, wet snow and a steady 20 miles per hour (32 kilometers per hour) wind, it's likely that other parts of that field will experience the same conditions.
Snow rollers can form any time there is loose, wet snow on the ground. In order for snow to be wet, it has to pass through a pocket of non-freezing air on its way to the ground. (Dry snow stays frozen solid throughout its entire journey.) At the same time, a stiff wind must blow while the snow is on the ground. In the western U.S., this combination tends to occur in February and March. In the eastern half of the U.S., those conditions can occur almost any time in the winter months.
Still, your chances of seeing a snow roller are rare. All elements must align for these whimsical snowy formations to emerge, so consider yourself lucky if you get to see some.