Stick with 5.5 and you will have plenty of power and you woln't have the risk of the machine bogging down with a bigger pad. Also although it is not required at first you may want to eventually grow your collection of tools and consider a 3in backing plate and pads for those tough to reach areas.
I've always used a 6" backing plate and 6.5" pads on my PC for the past 9 years with great and easy results .. 5.5" almost seem like spot pads ..what can I say I used to use 11&12" pads in the old days
