Sorry about the title, hard to put into one, concise sentence.
I'd like to know what a couple of the dimensions on fins do, namely width and thickness. That is:
- the width from leading edge to trailing edge and
- the thickness of say the leading edge.
I can see the diff between a formula fin and a wave fin and I understand that overall length will obviously give you more lateral resistance, so I guess width will too but what does the ratio between these two variables affect? ie how does a short, thick fin compare toa long skinny one, where they both have the same volume/area etc.?
How does thickness affect the performance?
So, let's say i'm looking to get on plane in light winds so I can (almost) definately sail every weekend what do I look for?
And feel free to recommend a fin 
(yes, more gear, it never ends does it?)
I can answer some (but not an expert). The measurement from leading edge to trailing ("chord") in itself is not all that important ....but the ratio of chord to overall length IS.
A long skinny fin generates more lift basically. That is why gliders (planes) have long skinny wings .... they generate a sh!tload of lift. A long skinny fin on a windsurfer: you will get upwind better, but they are not as turny.
However, what they will also do is because they generate more lift, you need less area to go fast and get upwind. Think about a wave fin at 22cm and a "pointer" at 33... they may be pretty much the same in area but do a way different job.
The rake of the wave fin and wide chord makes it manoueverable but the pointer will get upwind better (due to more lift).... plane earlier (due to more lift)... and may be faster in the long run as it has less area for the same lateral resistance (as that is what lift generates in part). That si why max sail on a wavebord with 22cm fin may be 5.7 and for the same area fin on a slalom board it may be what about 7.5m??
muddied the waters for you?
thickness also helps generate lift but is slower. Also a thick fin needs a lot of taper (in some cases) as you want a flexy tip sometimes.
In waves, a thick fin is nice for heavy guys as a "normal" fin flexes too much and can make the board track funny in a fast choppy bottom turn.
I agree with the MFC comment sorta.... but if you are racing you want a Deboichet I believe.
I've been getting into fins a bit this last year, and I don't think there's a simple answer, there's just too many variables.
But I'll have a go, this what I think happens but I don't have a deep knowledge, so don't take it as gospel.
The more curved the leading edge is, the greater the angle of attack you can have before it spins out at low speed. But it will also have more drag, and could spin out at high speed, because that curve produces too much -ve pressure at the front of the fin causing it to ventilate. So this sort of fin is more suited to dogging upwind.
the combination of width, thickness and length produce 2 other measurements.
Aspect ratio, which is the ratio between length and width.
and thickness to chord ratio, which is the ratio of thickness to width of an upright fin.
A high aspect ratio is faster because there's less flow from the low pressure to high pressure side at the tip.
the higher the thickness to chord ratio the more lift the fin can produce but also the more drag. Again if it gets too high it can be prone to spin out at speed.
I've tried short wide thick fins, because the weed here is long and grows from the bottom, I figured I'd gain more by getting the fin out of the weed than I'd lose, but I'm having spin out at speed trouble with the smallest, think it's a bit too thick (about 10% cord to thickness ratio)
The more lift your fin has, the bigger sail you can use and the earlier you'll get on the plane.
But once on the plane if the fin has too much lift it's going to raise the windward rail, making a spin out likely, which can lead to the front leeward rail grabbing and the inevitable catapult.
So if you want a fin size recommendation we need to know your board and sail sizes.
Yes Fin size is also very dependent on Board type.
The newer wider boards work well with the longer thinner fins while the older narrower boards worked better with the shorter wider fins mainly because the older boards dont have the leverage the newer boards have for example todays 110lt liter board could take a 45cm fin try that with an old type 110lt board you will break your ankles
.
As far as Brand choice Select fins are very popular and are reasonable value.
But if you want the very best yer Techtonics,Debs and there are the dedicated speed fins as well TimeMachine, Lockwood,C3 haha how much money do you have.
ta Vando
Oh I should meation there are a lot of good second hand fins around for about $100 so no need to spend a fortune unless you are trying to kick the WA,SA and the pitboy teams arses
Thanks guys,
I'm a 75kg very Joe Average, 110 litre freeride board, 7.2 m sail.
The primary objective for this fin is to get me planing in < 15 knots.
Yes, right, the glider analogy turned a light on. Has answered my question. Gliders have heaps of lift (60:1 glide ratio), not very maneuverable, where as a stunt plane has short, stubby wings. As you increase angle of attack the long skinny wings will be harder to turn than the short stubby ones.
(btw if you've never been in a glider do it. The first 10 minutes are the best thing I've ever done; really, truly flying. The mild nausea from then on kinda puts a damper on it however)
So... actually sounds like I need a long, skinny 38cm freeride fin (from the old RRD Justin)? Which I already, conveniently have.
nice djl,
i figured it must have been good cause some very good sailors tried it and liked it.
glad your enjoying it. 
