RE: too long fin
Severalthings will occur if you use a fin that's too long for the tail width and footstrap offset of your board.
First, the board will be hard to control at speed.
Why? Without the requisite footstrap offset, where you have good leverage and control of the attitude of your fin, you won't be able to control the attitude of your fin very well and other control issues (tailwalking, spin out) will occur.
Next, there is the issue of speed.
Too much fin is slow, as it's just added drag.
Combine this with inability to control the fin and you go even slower yet.
But this is not only about the length (span) of your fin, but you can also install a fin that's too wide (front to back, or the chord length)
and this fin, while it's about the right length, will have too much power
and since these wide chord fins are usually a thick foil as well, they just aren't very fast because they have too much area.
But they do solve the depth issues for some sailors.
If you have way too much fin for the size of your rig, then getting planing will be difficult due to the drag issues.
There are some other balance issues (rig size vs tailwidth vs fin size) as well.
Hope this helps,