A load of 1 oz of #6 shot has exactly the same recoil as a load of 1 oz of #8 shot. Shot size has nothing to do with it. Pay attention to 2 or 3 other things.
#1 The shot weight. Traditionally a standard 12 guageload had 1 1/8 oz of shot in any shot size. But you can buy shells with 1 1/4 oz, 1 1/2 oz all the way up to 2 oz or even more the longer shells. There are shells available with 1 oz and even 7/8 oz. The less the weight of the shot, the less recoil.
#2 How much powder. Traditionally this was listed as the Dram Equivalent on the box. This is an old holdover to the blackpowder days. If a box is listed as 3 Dram Eq, it means that it is loaded with the equivalent of 3 drams of black powder. If you see this information on the box, the bigger the number, the more powder and the more recoil.
Many companies are no longer listing the Dram Eq. and are simply listing the velocity of the shells in that box. You my see something like 1200 fps, 1300fps, or 1400fps stamped on the box rather than the dram eq info. The faster the speed, the more powder and more recoil.