[CentOS] Shrinking a RAID array

My google-fu appears to be weak today...

I currently have 8*4Tb in a RAID6.

So far I'm only using 6Tb
  PV         VG    Fmt  Attr PSize  PFree  Used 
  /dev/md6   Large lvm2 a--  21.83t 15.37t 6.46t

Let's say I wanted to remove 2 of these disks from the array and
shrink it down to a 6*4Tb

How would I do this?

-- 

rgds
Stephen