There are different ways to do this depending on which control chart platform you are running: With the Analyze>Quality and Process>Control Chart>Xbar Control Chart, simply change the subgroup size in the Constant Subgroup Size box (default is 5)
In the Analyze>Quality and Process>Legacy Control Charts>Xbar, there is a similar option Sample Size Constant:
"All models are wrong, some are useful" G.E.P. Box