The board is the right size (10' x 32" x 5") for our combined weight, 90+ kg + 15 kg.
Col Twyford sold me this board back in 2002 he made it from two blanks.
(It is a dog to surf). I would generally recommend short and fat for the board.
Especially if you can train the dog to the front.
The temperament of my dog is good, she positions well on the board. Not sure about your border collie.
The board now has its own deckpad on the nose, the fur and the wax just got to be too awful.
We stick to very flat water - the dog avoids white water, so I have given up on the idea of surfing with it.
It is pretty hard around my way to get a dog onto a beach.
See also:
everymanandhisdog.com.au/