The search radius is not needed for GLOBAL, it effectively uses a value about twice the distance between the two most displaced points in the dataset.
For other methods it is usually some fraction of this, typically 1/3 to 1/4 the distance across the area covered by the dataset.
Larger values can be used safely in all cases, but are needed if there is a lot of clustering. The search radius determines the connectivity between points and therefore needs to reach across the largest gaps in coverage that exist in a given dataset.
Generally it is appropriate to use a smaller search radius only when you know the data points are very well distributed or dense in comparison to the grid interval chosen.