For rule of thumb, th chain needs to allow you to reach 1’ past the edge of the part, and motors should be 1’ higher than the top border, and should reach to the bottom of the opposite corner. Add another foot or so for good measure. But that is my non-emperical fuzzy-crystal-ball-seat-of-the-pants guestimate. --grin–
For a 4x4’ area, you should reach at least an entire 5’x5’ area for each chain. So 5’*1.414 gives about 7’ minimum. For a 4x8 cutting area, it should reach across 5x9, requires at least 10.29’ ( =sqrt((5 * 5)+(9 * 9)) ), plus the slop required for hanging, and the tensioning lengths ( basically an additional 6’ for a 4x4, or additional 10’ for a 4x8 cutting area).
Again, my guess of an analysis, not based on any reality other than mine.
According to dlang (below), it should be 2’ extra around the cutting area, not just a 1’ buffer. The rough math remains similar.