York Template - Index Layout Display

Something I would like to modify in the York template is to have it so that the Index Layout displays consistently between Desktop and Mobile. Currently you can only choose between s/m/l/xl for Item Height in Index Layout section under Site Styles menu, resulting in either having the IL display properly in Desktop and cropped in Mobile, or vice versa. How might I go about writing a CSS code mod telling the Index Layout to be Xlarge in desktop, and small in Mobile? Any suggestions would be appreciated, thank-you kindly -