Hello!
I am an intermediate windsurfer who just bought the 129L Magic Ride ES (2023). I stepped down from a 151L Starboard Go. I was not wanting to go too low as I am still learning the manoeuvres and didn't go to the footstraps yet. The board came with a fin of 40 cm.
I want to use the board with a 5.0 m sail. I know it is below the 5.5 m size that is the minimum by design. Is it a real problem or it is just not gonna be optimal?
I cannot afford to have many boards. I have a second sail which is 6.0 m for lighter wind.
I would like to ask what fin size should I use? The one that came with the board (40 cm) as I understand can cover the 6.0 m sail?
What about the 5.0 m sail? Should I go for a smaller fin to not loose stability?
I was looking at the charts and they always show smaller boards and more narrow when going to 5.0m but if I am sailing in max 25knots is 129L really a problem? These charts are therefore not saying the fin size for my configuration.
It would actually be nice if the fin was fine for both 5.0 and 6.0 as I am guessing the 40 cm is also a lot for 6.0 and the one that came with the board is not G10 but softer and I have seen it easily gets sand abrasion in the spot I windsurf.
I would appreciate your help because I am a bit lost between sail size, board width and the matching fin size.
Cheers!
^^^
Yep, stay with the bigger fin. It will be just fine. The smaller and bigger sail will also be fine with that fin.
When you're going that crazy fast and have control issues, then worry about it. At your stage, bigger is better.
Ps , when you get your first plane, you will fall in love and want more. The answer to your next question, is yes to the 7.5m sail. 
The problem with too small a sail on wide boards (freeride, slalom etc) is you need to get planing and hang your weight off the boom to get out in the straps without sinking a rail. That cannot happen without sail power to hang your weight against.
5m is fine if going fast and getting into the straps.
5m is also fine if you are learning and just doing 5kn everywhere (that is not you)
If you are not getting in the straps, then fin size is not yet relevant. Just use it with standard fin and when going so fast you have no control go to the straps. You go to the straps because you have to, not because it is the goal in your mind.
Then when you do that a lot and cannot hold the rail down when planing, you get a smaller fin.
EDIT - +1 for Imax
Your size makes a difference too.
If you are closer to 100kg I would go minimum 6m.
If you are closer to 80kg, 5 could work.
Thank you all for making me less worried about the fin. I was reading about the problems with too much lift and was worrying it can make it hard for me to learn planning consistently.I was saying I have not yet used the straps but I do use the harness for a long time and I do plane when the wind allows. I actually need to move back touching the straps to keep control but I was too afraid until now to get into them. I know it is just a mental barrier, the same I had with the harness I guess. Everyone is telling me it is safer to be in the straps 
BTW I am ~85kg and I can see that with 5m i can go on a plane with around 20knots or a bit less. With the 6m a gust of around 16knots was enough.
Once in the straps, you can put all your weight on the harness, and make better use of the potential sail power.
If you do that and get flying, that's when the fin could become too big. It won't stop you from getting planing, in fact it could help. You need the lift from the fin, to counteract the sideways force of the sail, and convert it into forward motion.
So you'll soon find out when the fin gets too big, once you start going fast.
Straps are there to make control easier. Get in them as soon as possible, even if you then fall off plane.
I've seen beginners get all weird about them and it sets them up for all kinds of problems.
Did you put the straps inboard, it makes it easier. Start with going in the front straps, to do that you need to bear away and start getting speed, then sink your weight in the harness and move the foot in the straps without stepping too far back, otherwise the board will round into the wind.
It is difficult to go into the straps if you are planing already at speed, better do it when you just start planing.
It took me a long time to get into the straps when I was learning, had to switch to a seat harness, but when it happened what a difference !
Fin will be fine for any sail you want to use. You are a beginner and won't be out in extreme conditions. Try not to over think stuff. Get out there and enjoy the smaller board.
