If you’re creating records from all choices defined in the field’s configuration settings, then you don’t need s no matter what it is. The code would just be delivField.options.choices.length
If you’re creating records from each of a particular record’s selected choices, and s is that record, then you’re using the wrong syntax. To get a record’s cell value, you need s.getCellValue(delivField).length < That may return an error for you if the field is blank (meaning there is no array, not an empty array).
If s is a record, its saying undefined because delivField is not a property of the record. Records only have .id and .name as properties.