For the standard Shapeoko 3, I see that the max cutting area is 16x16. But what is the widest piece of stock that will fit without obstructing gantry motion?
The new style plate is ~4.875" wide — add that to get the maximum extent of the machine — you might get a bit more travel with the old style plate, not sure.
You may need to leave a bit of space to allow for the projection inward of the V-wheel bolts, ~4mm on each side to subtract.
There is an unofficial 3D model of the machine available: http://a360.co/1U9yxof (linked to from https://www.shapeoko.com/wiki/index.php/Shapeoko_3#Drawings ) and I believe GrblGru incorporates a full 3D simulation which may be useful for this sort of thing (linked to from the front page of the wiki).