This is the smaller trailer fin we use, the TT1 325
Then we have a 425 trailer, or 450, I remembered to look at the model of the little one, not the bigger. ;-) We also have some hatchet trailers, but its a very noticeable difference for riders that are learning. I have several times handed out a board, and the rider feels like it is to "squirly", so we put in the big trailer, and then they say it is too stiff, or you can see they just can't turn it well. Then you throw in the 325 trailer and it just stabilizes the board enough for them until they get better to take it out.
Jesse needs a rock stable board, and without the larger trailer, she couldn't have got up. I bet in a week or so, she will be trying out the smaller trailer.