Most ovens easily accommodate the 14-inch round or 14X16 rectangular stones. 

To find out what your oven can fit, you can open the oven and slide out a rack. Measure the flat surface of the oven rack from the back to the front. This is usually between 14 and 15 inches with some ovens achieving 16 to 17 inches.

Also, measure the width of the rack, although this is seldom the limiting dimension. Be aware, most oven racks have an upward slant in the very back of the rack that prevents the oven rack from being pulled out when the oven rack is slid all the way out. Please don't use that part in your measurements.  The stone will not fit flat if you try to push it up on that portion of the rack.