Yes, smaller, lighterweight sailors can use smaller fins and smaller sails for a given windspeed than larger heavier sailors, but even big sailors tend to use the smallest fin that works for the conditions.
Using a larger fin just adds more drag.
Since you will be using weed fins, and Starboard really gives you no size range applicable to weed fins (they tend to have alot more area than a similar depth (span) vertical race or pointer fin.
What boards are you looking at, and what sail sizes do you think you will be using.
Also some indication about the windspeeds you plan to sail in.
At 57 Kg. I don't think you will be using fins much smaller that those recommended in the range for each Starboard board, and if you will be using weed fins exclusively, there are many different types of weed fins, and some can be sailed very small, others you need a little more size.
What weed fin brands are available in your area?
Hope this helps,