As a rule of thumb, your SParms size should correlate with your Polo shirt size (both for males and females). I.e. if you wear a size Small shirt, you will be a Small in SParms, etc.
If in doubt, err on the side of being smaller as being too big may lead to the top of the SParms slipping down marginally.
Please use as a guide only:
SParms SIZE CHART
SParms Plus SIZE CHART
SPglove SIZE CHART