The field drawings contain the tolerances, which vary based on the field element.
Check out the field assembly document.
The overall length of the field is +/- 3", because that's a dimension that's incredibly difficult to get perfect, and impossible to keep consistent over the course of a 3 day competition.
Measurements from a wall to a piece of tape (and tape to tape) are +/- 0.5", and those are easier to keep within spec.