the wj has a 60 mil width, so on 1/4" i'd just give it a full width and budget like 120-180 mils for the cut. a cool thing that they have with their 2-axis is that it has programmable taper up to about 3-4 degrees so that cuts are straighter than a typical WJ, but on the flip side, the jet theta is constantly moving to correct for concavity, and that kills a lot of absolute precision. looking for tolerances that are any smaller than you absolutely need is just going to cost you a bunch of money you don't need to spend.
for tolerances, my guy really needs to see the drawing. for example, jd and i gave him a drawing with a font that had curve interpolation already applied. had we printed this, it would have destroyed the tolerances we were looking for, because the feed rate of the jet is pretty slow and the thing has HUGE amounts of inertia. if you give it an interpolated curve (ie, a curve approximated as a finite number of linear segments), the jet head carriage will judder like crazy and destroy your stock.
i should be able to find a 20 x 10 plate easy. you may have to go thicker though. 0.3 seems to be a pretty common thickness for the droppings at IMS.