Hopefully the free size is not less than the 4 gig MS pro i've been using for the last year.

But im gonna wait til Sandisk releases all 3 of their 4 gig sticks before i decide on which one to get. I have to use an adapter to get the MS pro stick i haveto work, and since it attaches to the back (on the left side of the PSP) it gets in the way of my hand. I definately would rather just have a pro duo that fits right in with nothing sticking out.
(For anyone interested below is a pic of the adapter, you can see how it gets in the way...)