Header way too big can I reduce the size:

Have you asked on the theme’s forums?

https://www.markettheme.com/forum

Commercial themes are not supported here, particularly non-GPL themes which are not consistent with WP philosophy and policies.

This may also be helpful:

https://www.chipbennett.net/2010/12/10/only-download-wordpress-themes-from-trusted-sources/

Thxs 4 the speedy reply
Yes I have posted on the Market Theme Forum got one reply saying resize the header image but it is Sunday!
I’m hindered by a total lack of knowledge what or which file sets the height of the header or is it a standard size? I suspect that the theme dictates this but there has to be a line of code somewhere that sets the header height
May not be able to change it but could put in a product image rotator?

As I said, that theme is not in keeping with WP GPL policy and values — so you won’t get any help here for it. Sorry, but you really might want to consider another theme.

That said, try using Firebug to work with CSS which generally determines appearance on the page.