Generally, the minimum dimension of a runner cross-section should be 1.5mm greater than the thickness of the part in millimeters. This enables the cavity to pack evenly and produce an even volumetric shrinkage.
The selection of runner dimensions may be limited by the type of material and the design of the part. For example, a 1mm diameter runner of styrene may snap when the part is being ejected from the mold, but a 1mm diameter runner of nylon may flex. The results also depend on the type of ejection system that is used.