Hi
I'm running a tubeless AWW 16" with a 1" axle in the front (2000-up). I went for the stock size as I got a good deal on them, plus I like the balance of two same sized wheels. I am very happy with those wheels and whenever I bother cleaning them they look great with the fat spokes. No issues with air leakage and the chrome seems solid.
I would be a bit sceptical to installing a 21" up front although I've never tried a tourer with such a set-up.
But, I have been riding both the FLSTC (130 x 16") and FXSTC (100? x 21"), and to me the Heritage feels much more solid in the curves and when braking hard.
Another thing worth considering 'while you're in there' is to install a 2002-up swingarm (with spherical bearings instead of those soft cleve blocks). I did it last winter and the difference is pronounced. I can still feel some flexing, but it's now got a tighter feel to it. Next step is test out some aluminium bushings to stiffen up the rubber/steel donuts.
YMMV, of course...