The "published" fin range is the range of fins that one would normally use in this board
based on an average 80 Kg. sailor.
The "published" range is arrived at based on the width of the board, and alot of testing to see what is too much and what size is too small (again for an average 80 Kg. sailor).
The test team does include both heavier and lighter sailors than the 80 kg. so it's a fair
Since you are much lighter, as agrelon suggests, you can go with a smaller fin for a given sail size, but fin size is always "speculative" in that if you are a bit underpowered you can help the situation with a larger span (length) fin to get you planing sooner, and conversely you can use a smaller fin when overpowered and the board is beginning to get hard to control to get some control back and keep on sailing.
Normally if you are overpowered, it's good to change to both a smaller fin and smaller sail size.
At 57 Kg. you do not fit in the "average 80-85 Kg." sailor category, so the published fin range may not mean very much for someone your size.
Since you are trying to buy fins, it might be a good idea to purchase a fin in the stock size for 2009 and then work up and down from there.
The "stock" fin was always slightly smaller than the mid range for the board.
Some sailors used much larger (like 8-10 cm) and many used smaller (like 4-6 cm) depending on sailor weight to some degree, but wind and water conditions to a larger degree.
Hope this helps,