Bill,

The standard "textbook" recommendation for the optimum screen frame size is 6
inches from the edge of the squeegee to the internal diameter of the screen
frame. For example, if your stencil width were 18" and your squeegee blade
length were 20", then you would want the screen frame width (inside
dimension) to be 20" + 6" + 6" or total of 32". Generally, the bigger the
screen frame the better because you will get less stretching of the screen
fabric during the print stroke when the squeegee is pushed down against the
substrate.

One of the best "experts" of screen printing and screen frames that I know of
(they sell Diamond screen frames) is:

E. W. Dorn Co.
7831 159th Place, N. E.
Redmond, WA 98052
Phone: 425-881-5419

Hope this helps.

Larry Fisher

Allen Woods & associates