If you have acquired the 'Custom Fields per size' add-on module, the option 'Diff. per size' will be visible.
This allows you to set up which of your style custom fields should have different values per size.
To enable this option, click the checkbox the 'Diff. per size' when you set up your Custom Fields.
When this option is sat it will be possible to give differentiated values for any specific custom field on a style - read more about it here.