Custom Logo Size

Hello, My website is http://salesbrainwork.com I'm trying to use a custom-sized logo but the image isn't rendering to the correct size. The theme is shrinking the image. I would like to have something around 420x100 or 315x75 Something that looks proportionate to the site. See the following site for an example https://www.thinksales.co.za/ FYI that I'll be upgrading to the PRO version soon. Thank you