First of all I am not an expert or a sailmaker so this is just conversation. The spinnaker is one of my favorite sails. I had angst in the beginning and it can be tough to handle when things are going wrong but I fly it whenever I can now.
If my High School
Math is correct we have a 39.7 foot mast
, a 41.9 foot forestay and that makes the J dimension about 13.4 feet. I think better in feet.
I have read that about 1.5 times J is a good start point for the spinnaker foot. So 20 feet. Converting back into mm that means about 6,000 mm. The breast will be wider.
There are times when you will want more power and there are times you wish you had less. Depending on the cost of the sail (unable to buy 2) I would try and go sailing with someone who has a similar boat so you can get a feel for it.
Here are some thoughts I would have regarding power -
Crew - Is it just 2 people all the time? If so I would go a tad smaller.
Boat weight - If the boat is heavier I'd go a little bigger
Reaching - A larger sail will be more to handle when reaching*.
Reaching - I agree with mark on luff dimension. I think luff = mast height is about right. You don't want the sail in the water
when calmed or when reaching.
Wind - What winds do I expect most of the time. If higher winds I'd go a tad smaller.
(*When reaching you are likely to have the pole low and forward and be sheeting hard)
I think your stated choices are
10600 X 5800
12000 X 7200
12200 X 7400
Given those choices I personally would go with 12000 X 7200. It is bigger than I stated but the 10600 X 5800 might be too small most of the time. We are also talking about the breast dimensions you provided not the foot dimensions - The 7200 (23 feet) will be narrower than that in the foot.
I think with 2 up cruising the 12200 X 7400 is too big. Not so much the foot but the hoist height of 12200.
But I am often wrong...