This question is a bit unclear for us. Could you please clarify us with more details where exactly you want to change background-size?
Also please provide us with link to the page and wp-admin access in Private Content.
well when I set an banner background via theme option there is a field to set background-size. Is it working and if yes how ? I have tried to set values there but it was not taken into account… I can change via custom css though.
Thanks in advance,