Hi. I'm quite new to all this but I had a similar problem on my site where I wanted the logo on the footer to be smaller than the header logo on mobile. I found a way of cheating it by adding some extra transparent space to the sides of the logo .png file that appears on the footer. That way when the logo image is displayed full screen on mobile it looks smaller as it includes the blank pixels to the sides. It takes a bit of tinkering back and forth to get right tho. If you want you can then adjust the content width of the footer block and/or add spacers so the footer and header logo appears the same size on desktop but smaller on mobile. May not be the best method but works for me so far!
