The size difference in the Silva vs GSP fight would barely have an affect on this fight imo...
First of all GSP is bulking up getting ready to move to MW already, he is even bigger then when this video shows.
Second, GSP is probably the best wrestler in all MMA. Size advantages for strikers is not nearly as beneficial as size advantages for wrestlers. That is why say Penn can defeat almost all his opponents(even those that are larger then him) except for GSP because of his wrestling. Anderson doesn't have that advantage.
Third, Anderson doesn't have godlike take down defense or wrestling ability. Almost anyone that has gone in with a game plan to take him down has been able to get him to the ground.
This fight would be a lot closer then some people think and I personally would pick GSP over Silva. He has a height disadvantage and a slight reach disadvantage but other then that he is the perfect match up against Silva. He has very good striking(he outsruck Penn and Alves to name a couple very good strikers), amazing wrestling and cardio, very good submission knowledge/defense.
GSP is the only fighter in the UFC not in the HW division that I think could beat Silva.