PP size is specified when you create a volume group. As far as I know, it's impossible to change PP size after a VG is created.
Since it happens to be rootvg, it's impossible to change that value without reinstalling (or mksysb'ing) the OS, and when you do that, you don't get the option of specifying PP size when rootvg is created - PP size is calculated by the installation routine to use as much of the specified disk(s) as possible and keep under the 1016 (or whatever) PP's per PV limit.