I never touch my 6.5" pads. 6" is as large as I go especially with my
UDM. A 5" backing plate with a 6.5" pad has too much room around the edge and you end up using only 5" of the pad anyways. The extra 1.5" that has no backing is worthless (PC & UDM). It is very hard to get to any edges of the car because the sid eof the pad raises up. Also, if you want to do any correcting without marring, a 4" or 5.5 or 6" pad is needed.
Now I am sure that there will be plenty to argue with this, but for a beginner the smaller pads will show the most work. A beginner needs to see pads and products work to learn from. A 6.5 inch pad and 5" backing plate is very very difficult to break down the polish and show the beginner the three steps that polishes break down to. So if you are new to this game, a 3.5" and a 5" backing plate is essential as is 4", 5.5" and 6" pads. They will give you the biggest bang and speed up the learning curve. IMO (which probably isn't worth a damn)