X BAR - RANGE chart - Subgroup question

From the xMII 11.5 help on SPC charts -

http://help.sap.com/saphelp_xmii115/helpdata/en/Advanced_Topics/Illuminator_Quality_Portal/iSPCChart_Editor_Overview.htm

SaleAmount OrderID CompanyName ShippedDate

2556.95 10393 Save-a-lot Markets 01/03/1997 00:00:00

2505.60 10398 Save-a-lot Markets 01/09/1997 00:00:00

3868.60 10401 Rattlesnake Canyon Grocery 01/10/1997 00:00:00

2713.50 10402 Ernst Handel 01/10/1997 00:00:00

1) If I were plotting the saleamount, what are the considerations for choosing a subgroup size for plotting a "X Bar - Range" chart for the ABOVE sample data?

2) If I were to select a subgroup size = 3, (and therefore consider the first 3 items as one subgroup) would this be a correct method of selecting the subgroup size, since the variation within the subgroup ie. between item 3 (Saleamount = 3868) and items 1 and 2 (Saleamount approx. 2500) is large?

Appreciate your ideas!