One of the Sportster models comes with a 21" spoked wheel stock. Not sure if it's a good adv tour wheel, but it's already there and I'll probably try it.
If I understand this thread so far, a 21" wheel is not ideal for the road, especially the twisties. It's probably okay for the highway and non-aggressive riding. The narrow contact patch may want to follow road grooves, or it may not, or it may depend. The 21" wheel/tire will have greater diameter, even though the tire appears to be lower profile than the 19 or 16 inch wheel's tires. The Sportster is too heavy to be lifting the front wheel easily (unless I so overload the weight behind the rear axle, like they did in the Cannon Ball Run Movie....). There is disagreement on the advantages of floating over loose material or slicing through loose material, with slightly more leaning toward slicing through.
I'm building this for long highway trips to off pavement destinations, like Prudhoe Bay and the Trans Canada Ride for example. So, I should consider the importance of safety on the pavement. Is there a braking reduction with the narrow contact patch of the 21" wheel's tire?
In case this little bit of history helps any, Most of my long rides this decade have been on big heavy touring bikes with fat front tires. They just glide down the highway effortlessly. Haven't taken these 900lb monsters on too many fire roads, but they did surprisingly well on the few bad gravel roads I had to take, and on the few patches of dirt my shortcuts demanded.