About your Die Kriegskunst query...
We've played about a dozen games of this. It helps if you've already played General de Brigade or some other member of the rules family.
On the plus side, these rules have a really good command and control system, and the combat/morale results seem likely for the period. However, they do have a fair number of charts and are not fast-play - if that's what you're looking for.
The big problem with Die Kriegskunst specifically is that it is rddled with typos and has a number of omissions. Despite promises, the authors have never produced any errata. So, I could only recommend them if you want to play detailed games with each player commanding a brigade or so, and at least one player is familiar with the GdB system. That said, we've enjoyed the games we've played.
Figure size - we went for 10mm because I saw a painted Pendraken Austrian battalion at a show some years ago, and thought they were beautiful. With big battles, the long lines of troops look just right.
As to other rules, we play Age of Honor, the F&F offspring for big historical battles, and mostly Rank & File for battalion stuff. We tried Black Powder - gave it a good go, 5 games - and decided not to. It was described as a toolbox - but the box was coverd in sequins, the screwdrivers were silver-plated tin, and they didn't bother to include a hammer. I suspect that the success of this set is mostly down to the fame of the authors and a really good marketing scheme.