changing header specifications

Please don’t refer me to documents Designing headers – i’ve already looked at them and they are not clear to someone who is an absolute beginner like myself.

I’m currently using the kirby theme and the default size for a header is 940 x 198 pixels. However the header that I have created is 750 x 500 pixels. I do not want to re-size my header to fit the default. I would like to find out how to change the css settings to allow my header to fit.

Any help would be most grateful!