The logo is determined in the layout.css file. The logo is set in multiple locations. I'm not sure which layout you're using, so here are the CSS selectors for Charcoal_Grey. But the Azure_Blue has similar selectors.
body.media-desktop div.header div.logo
body.media-landscape div.header div.logo
body.media-mobile div.header div.logo
You have to update the sizing for all three selectors to target all devices. Please note that the title in the header is expecting a logo of a certain size. If you change the logo sizing, you may need to update the positioning of the title as well. For mobile, please check: