I think when you say you want a knife that is "useable", most makers here take that to mean "practical". After reading your posts, I think you just want one that is more or less a large scale version of a "functional" knife - the difference between a full tang Carbon steel sword with proper heat treat and tempering, and a chinese sword with a thin welded tang and none of that stuff. The problem is that a disproportionately large blade design also makes a knife dysfunctional, for a lack of a better term.
Nick Wheeler had a very good point about the handling of such a blade - but even Sengvin's blade has a more suitable handle than your illustration, at least from the angle it is photographed. Joe also brought up some good points about a good distal taper and the balance. Remember, an unwieldy blade is every bit as dysfunctional as a blade that doesn't have proper heat treat. I think the best you could do is to commission a maker that has some large bowies in their portfolio to make the largest functional knife they can. A totally functional knife will retain its value better, and I think in the long run you'll be more satisfied. A heat treat and proper construction will not make an unwieldy blade any more functional. As you can see though, a competent knifemaker can make you a very large Bowie that approaches your specifications that will be a functional knife.
Of course, I also understand DeRosier's post; you might actually just want a huge block of steel to hang on the wall. If that is what you want, more power to you, and I hope you find someone to make it for you. The drawing you made reminded me of one of those Final Fantasy swords from the Big Sword video. There are probably more people out there that would purchase a 8lb block of steel attached to a broomhandle than most members here could comprehend, but there is a lot to be said for a knife that is both large AND functional. Just reconcile in your thoughts that heat treat and proper construction won't make it any more functional in reality. There is a nice compromise that is very close to what you're wanting though, and a competent maker with experience on the larger side of the Bowie design could guide you into finding it. If 3" and a slightly different handle design on a properly ground bowie is the difference between having a dysfunctional blade that isn't any good for anything except hanging on a wall and a knife that is in fact functional (you're paying for the heat treat and such anyway), why not make that compromise?
Yeah, this is the main reason why I posted here -- in search of some tips on a maker who could make such a large blade. If it was possible to make such a large blade practical and functional, that'd be great, but I would assume that would require someone extremely skilled. In any case, I meant useable as in it wouldn't break or bend - it'd be as strong as a regular heavy duty knife. As long as the lack of functionality doesn't give way to a weakened knife, then great. I would like a knife as close to the specs as possible, but I'd of course be up for letting the maker widen the parameters.
The drawing was just meant to illustrate some of the basics I wanted; the proportions of the handle are way off, for example. While they are the specs I would prefer, they could be considered as the extremes of what I would like. While this would just be a (functional.) display piece, if a maker suggested that slightly reducing the length, width and/or thickness would improve the balance and handling, and make it a lot more practical, I wouldn't have anything against that - so long as the specs aren't that far off the original marks.
If, as you say, there's a 3" difference, then that'd be great, and I'd let the maker reduce the length. As I said, I'm looking for someone who could make a large bowie, and has experience with making larger knives. I would give the specs to them, and listen to what they have to say. I wouldn't mind reducing any specifics. But what are the price ranges? Ie, at the most basic - how much would it cost to get someone to heat treat a 20", 0.4 inch bar of knife/sword steel, and make it resemble a bowie?
However, now that I've seen the knife by SENGVIN at a different angle, I think it's perfect, though. I didn't realize that it was that large (I only had a top down picture of it.). Since this would just be a display piece, I wouldn't mind an even thicker blade, but that's pretty much exactly what I want. (Without the text and the cutout, though.)
Hope you don't mind me posting this, SENGVIN?
These are the specs:
spec:
-blade 12 7/8
-overall 18 3/4
-thickness 5/16
-steel D2
-RC 58-59
-Full Tang
-maker Gil Hibben