Hi, bit of a newb question here. Can someone please explain to me how board recommended sail sizes matter. I've got a Severne Dyno 105 and a JP Magic Ride 129. The Dyno has a recommended sail size of 5m to 7m. What exactly starts to go wrong when you go outside these recommended sail ranges? For example if I sail my 4.7 sail on this board. Given my limited experience I'm finding it hard to differentiate between poor technique and incorrectly fitted equipment.
Thanks.
It's a bit easier to go smaller than recommended than larger. I have a dyno 115 and have ridden it with a 5.0 some and even my 4.4, but it needs the mast base more forward than with the 5.6 and 6.3. Min recommended is 5.5 I think. If it's not forward enough the fin or fins lift too much and I bounce some a lot when wound up with the small sails. Smaller fin would help too.
When the sail is at the high end of the range (7.5 max), it really needs a larger single fin and straps outboard. If not, it's hard to sheet in, cramped, and the fin won't take it that well. I have gotten the 115 planing with an 8.0 even but it really needs a bigger board and fin. Just feels very off and suboptimal. Will struggle to get upwind if not lit up and will spin out a lot if it is.
Recommended sail sizes are the range of sizes that the manufacturer has deduced work best with that size board. Other sizes may work OK, but will not be optimal. In fact, I find that each of my 4 slalom boards works best, (is optimised), with only one size sail for me. If I change sail, I almost always change to the matching board. (All cammed Race/Slalom sails) ie: For my 175cm and 73 KG its IS80/5.7m, IS87/6.3m, IS97/7.0m, IS117/7.7m. If I use the smaller sails on the larger boards it feels less balanced, but mainly, if I go to the smaller board with the smaller sail, I go faster and I am more balanced and comfortable. Using the larger sail on the smaller board does not get the best from the larger sail as the board does not have the volume and planing area to be as efficient and carry the larger fin nessasary to get the best from the larger sail.
But, if it was the only gear I had, and it was the only way I could go for a sail, the biggest and smallest mismatches in the above gear would be sailable, and I could probably enjoy a bit of fun. 
When it comes to Freeride gear and Wave gear, the situation is similar, but the emphasis is more about having the volume and fin to carry a larger sail on a smaller board. Smaller sails can work better on larger boards, as areoagnr says, but then the boards can be too large for the amount of wind (which is why you want to use the smaller sail) and chop/bouncing and control (windage/lifting) become the issue. This can be less of an issue on flat water.
So there is Optimal, and most efficient. And there is just getting out and having some fun with what you have at hand. 
Along with volume, a board's max width + tail width affect how well it can carry a sail within a certain wind speed range.
105L is usually 4.7-7.0 something like that. 4.7 will need a small fin while 7.0 a long one. I've used 4.2 on a 105 when winds were between 5 and 30-40 knots. I also used 7.0 when it was around 16. My particular board is a freeride board great in the waves. It worked better in the smaller sizes
A friend of mine used his 105L with an 8.0 and liked to sail powered up. Being powered up lets you ride a bigger sail on a smaller board to cope with wind lulls.
Similarly a smaller sail (for the wind) will benefit from a bigger board to get planing.
