My top bars are also 19 inches, and it works well to be a standard size I think, even though at 3/4 inch stock, they are a bit thicker.
As I said in previous posts, I made the frames in preparation for cutout comb that I was unsure of how to mount. If I were to have a cutout to mount today, I would make the frame much shorter since the bees don't seem to typically make combs so long in confined areas, and it can always be trimmed. As can be seen in the photo, the bees would rather start over from the top than to build upward.
Regarding attaching comb to the walls, I read somewhere that the angle of the wall makes the bees think it is part of the floor and they will not attach comb to the floor. Mine is still a work in progress, so I don't know if they will attach to my walls or not. The angle of the walls of my hive is 24° from verticle, per the plans I (mostly) followed. Google the Barefoot Beekeper for the plans by Phillip Chandler for reference.