The brochure says a width of 20" in front, 18" in the back. So I'd go 17-17.5" or so E/W.The smoke: yes. The paint made my basement"look blue*. This will be common for each fire that is hotter than the previous ones. Just follow the manual, by doing increasingly large fires. Open the windows (2 opposite if possible) while you cure the paint.
The other questions I have to defer to others. But the split lengths should be in the manual or brochure that you can download from the BK site.
Depth (N/S) is 16.75", so don't go larger than 16".
If you cut all your wood to 16", you'll be fine.