Can vary from about 6mm to 25mm. Maybe more for a preliminary screen, depends on job specifics.
Also important is the design - whether it's 1 dimension (bars) or 2 (mesh). Nowadays 2 dimensions is often specified as it does a better job. On a bar screen objects much larger than the nominal spacing can get through e.g. plastic backing strips from "feminine hygiene products" (to put it coyly!)
Cheers.........Codey
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