The Set Average Cluster Size option is found under Advanced Options in the Space Filling Design red triangle menu. This option enables you to specify the average number of randomly- generated points used to define each cluster or, equivalently, each design point.
By default, if the Number of Runs is set to 200 or less, a total of 10,000 randomly generated points are used as the basis for the clustering algorithm. When the number of Runs exceeds 200, a default value of 50 is used. Increasing this value can be particularly useful in designs with a large number of factors or where disallowed combinations restrict the distribution of points used in the clustering algorithm.
Note: Depending on the number of factors and the specified value for Number of Runs, you might want to increase the average number of initial points per design point by selecting Advanced Options > Set Average Cluster Size.