RE: HELP fin size
The stock 38cm will be a good all round option for 7.5m conditions, for the 5.7m you really wil be better with a smaller fin in the range 30-34cm.
The actual size of that fin (especially) can vary +/- 2cm because if you take a more high aspect fin, for it's range (performance size) it will be longer.., or if you take a lower aspect freemove style fin (can be a good choice for playing around and moves !) then the sizing would be smaller, so around 30cm (or even a little less).
You can also consider addition sized fins, but fo rthe combo you described the one extra fin would be a good point to start at.
Please let us know if you would like more info etc.
Cheers ~ Ian