Changing Logo dimension from square to rectangle

I have an issue on installation of website around the header and logo – on which the ‘customise’ menus don’t seem to allow customisation.

1) Our logo is rectangular, but the default on Customise for logo is a Square box, and that’s it. How can I change this? When I insert the rectangular logo in a square box it makes it much too small, PLUS it leaves a large boundary above and below, e.g. before the tagline (I have unclicked ‘show title’ as it doesn’t allow me to change the appearance significantly, for instance even if I change font it is all capital block type. Surely one should be able to change this, but how?

How can I change this to a rectangular box, AND create a custom size?