I have not built the sailrig but bought the plans and it is likely that I will build it in the future.  I am also an avid sailor for what that is worth.

Certainly your sailing experience level is important but so are the conditions where you intend to use the rig.  If you are in a windy location like Chicago or San Francisco, I would go smaller rather than larger.  If you will use it inland where there is less wind, then go larger.

Another consideration is reefing.  You can reef the 70 sqft sail down to 40 sqft but you can't ever make the smaller sail larger.  Not sure if reef points are standard on the sails but they are easily added by any sailmaker. 

Another consideration is the rudder.  Everything that I have read, including some written by JC Harris, indicates that a kayak rudder system is insufficient for the sailrig, almost regardless of sail size.  The sail rig has optional rudder kits, either foot or tiller controlled.  I strongly suggest one of those. 

Looking forward to pictures when you get it on the water.   

   Thanks Mark. I'm near the ocean in Delaware so we have our share of wind but calmer in the bays. Good point though. 
Not sure on reef points but also I'm seeing mast height on 55 is 3 ft greater than 40

so question on stress on the hull and overall stability? Is it worth it for what i might gain.  Leaning towards the 40 not looking to win any races. PP

Structurally, installation of the 40 ft^2 and 55ft^2 sails are identical except for the longer mast.  The 70 ft^2 sail requires more structure/work.

From a stability stand point, the 55 sail reefed down to 40 ft^2 of sail area will be virutally identical to the 40 sail.  The only difference will be a slightly heavier mast with a slightly higher center of gravity.  With a 10' beam, that will not matter in the slightest. 

From my perspective, you would be wise to go with the 55 sail because it will be a better sailor in light winds but just as capable reefed in stronger winds.  The only real downside is slightly higher cost. 

   I don't think the stock 55sf sail has reef points as standard. The 70sf sail comes with two sets standard and the sail attaches to the mast with slides instead of a sleeve for the 40 and 55sf options.

I remember a comment on another thread with the 70sf on a WD12 that said that with any kind of a breeze the hull would "squat" and required early reefing. I haven't seen any numbers on how much each reef reduces the area of the 70sf but it looks to be in the neighborhood of the other two options.

As with just about everything else, it comes down to cost. The total for the two smaller options,I've read, is a little north of $600 but for the 70sf it is more like $1300. I don't know if that includes the $400 reinforcing pachage but I think you would need the larger rudder pachage to control it. The mast for the 70sf is made of tubing that is twice as thick as the other two and requires the instillation of slide rails.

Keep us posted. I have a WD12 that was my first and is built like a brick chicken coop because I'm not exactly svelte. I would love to add a sail rig. I need to finish a gazebo first which is a prereq for any further boaty like projects.

I am building a sail rig, and haven't bought a sail yet. I've been wondering about this too, but the smaller sail is described as "neck snapping" acceleration on a single kayak, so that's what I was planning (although in my case I'm looking for a non-stock option)

   Thanks guys. I'll be ordering today or tomorrow the latest before the discount expires. So I'll be calling CLC for some answers. Keep you posted 

Also any thoughts on optional boom? Necessary or not advantage or not?

@ Bob what do you mean by a non stock option? What rig CLC? What mast came with it?

Also the 70 is not on my radar  Cost, extra work and I think overkill for 12 or 13 single.

   @ catboater. Good point I'm only 165lbs

   @ PdP

I would consider having a boom. I had a Hobie Mirage with a sail rig and down wind the sail would fold up quite a bit making me long for a boom. I don't know if it would have made much difference in performance but I would have felt better.

   So decided to go with the 40 ft ( boom included). 55 not available with reef points I'm sure I could have gotten done somewhere but not worth it for cost and I was told the 40 should be fine for lightweight kayak and lightweight me by the ocean. So ready to build on! Be sure to post pics when I'm done. 
Thanks all PP

My sail rig is fitted to a WD 14 and I have the 55 sqft sail. It's enough for me because of both cost and power for the boat. It can't be reefed as it has a sleeve the mast fits into so I'm sure there are limitations in the performance but speed isn't my main consideration, I just like to get out on the water.