Re: fins on wakeboard
As has been said...personal preference. Everyone should try experimenting with fins of different sizes, and with/without fins. It DOES make a significant difference, and every board is indeed very different.
Just an example of my personal experiences
I have an Obrien board that I HATE, and it has 1.5" fins. My buddy loves it, so I leave it alone and let him ride it.
I have an old Origin board that I absolutely LOVE so much that it's cracked and worn out desperately needs replacement, but I still ride it. It has 1" fins and I never changed them.
I recently got a Byerly board that came with 2.25" fins, and could do almost NOTHING on that board...not one surface trick (except faceplants LOL), because the fins were way too big. So I took those 2.25's off and tried riding it without. Absolutely the most slippery thing in the world...couldn't stay up on it, and almost couldn't catch a good edge unless I got REALLY fast and aggressive...then it worked. I'm learning this is a very technical board and it will take some time for me to get used to it...seemed like every time I dug my heels in to catch an edge it just slipped out and I ended up on my tail. But if I dug in faster/harder I could actually catch that edge before it slipped and save it. So I felt I needed the crutch of some fin. So I put the 1" fins on it off of my Origin and tried that. Better, but still too slippery for my taste and skill level. So I took the 2.25" fins and trimmed them down on my workbench to about 1.75". Better, but maybe sticking too much. Now I can at least grab that edge without the board kicking out on me. I might take another 1/4" off but I'm gonna ride like this for a bit and see if I get used to it.
Experiment, adjust, experiment, adjust, and FIND YOUR OWN preference.
As a very generic rule, beginners who don't have good board control will find that good sized fins are very helpful in stabilizing. Definitely put them all on to help you learn how to get out of the water, and how to just generally ride around without falling. Once you no longer are randomly falling at every little ripple, and after you actually learn to use the EDGE to turn instead of the fins, you may not need the fins anymore, and you will probably want smaller or trimmed fins because less fin generally makes many tricks easier. It is very important that beginners learn early that they should NOT rely on the fins to turn the board. You are supposed to turn the board by using the EDGE of the board. Therefore it is a very good training tool to take your fins off to eliminate this crutch. If, without fins, you find it is more difficult to turn, it is because you've been turning WRONG with poor technique. Indeed it will feel much more slippery, and you will probably fall more often, but it is an outstanding training tool to take them off and give it a go. Again, this all varies with the board design.
Everyone has their own happy place with regards to fins, and every board is VERY different and reacts differently to fins...therefore this question simply has no fixed answer.