IME plakats in general grow a little faster than long finned breeds, but given equal care they end up at about the same size. that said, you're more likely to have a trace of giant genetics in plakat lines than other breeds, so some of them may grow bigger because of that.
EDIT: just thought i should throw in that all of our biggest bettas have been either plakats or VT's.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)