First time shaping need advice

I got a 6’9 blank and I wanted to do longer retro fish(6’7), but I’m not sure what dimensions I should go with, any advice?

More info needed
What are the dimensions of your current go to board.
How heavy are you.
What type of waves are you mainly surfing, beachy or point, if it’s a point you can go for more volume, whereas at a beach break, more duck diving so less volume preferred.