In a volume group there is a value LTG, which indicates the max IO-size that will be used for any disk in that volume group. You can tune it, but shouldn't set it higher than the max transfer size of the disks in the volume group. I believe VG creation will stop you from choosing a too high value but not sure about that.
That said, AIX's LVM/VMM will pretty much cope with wathever an application throws at it (will split it up in multiple reads if it has to).
I guess it makes sense to have a stripe size equal to the max transfer size of the disks (i.e, the LTG size of the VG) but that is just a gut feeling.
google for LTG and stripe and performance and read on...
HTH,
p5wizard