Thanks for the replies guys! I am not planning on making it a solar kiln at this time, but I wouldn't rule out a conversion sometime in the future possibly. For now, I am going to use a conventional dehumidifier, fan(s) and heat lamps or such. I think I am liking the 12' length...as I will most likely have most boards prepped in the 10'-11' range, so that would leave me a little wiggle room(air flow) on the ends too.
I don't have a mill now...but that is why I am preparing to make a kiln...I plan on buying a mill in the next 6 months or so. Although I don't live on it, I own a farm with enough timber that I will be able to harvest more than enough of my own timber to supply myself and my sons with our own wood. Along those lines, I have never really done much furniture making with walnut, but I intend to harvest a walnut or two on my place and dry those boards and use them eventually, along with both red and white oak.
Also, while I have a couple of longer trailers at my shop I could use, I envision using my "regular" trailer to bring wood back to the house from the farm and it is 12' long...so it seems like that would be max length for my boards.
FWIW, I was going to store fresh cut lumber outside the shop addition to air dry and then bring it in to finish it in the kiln.