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How to change the size of a logo?

You can set the width and height of the logo:

You can use this file:

your-domain.com/templates/less/header-styleXX.less (line 30)
So: 
// Logo
.logo-type-gantry #rt-logo {
    background: url(../images/logo/style1/logo.png) 0 0 no-repeat; width: 231px; height: 52px; display: block; margin-left: -25px; margin-top: 23px; margin-bottom:12px;}

Also, 
Important Note: Always edit the .less and not the compiled .css files that are output to /css-compiled/ folder

You can review these examples to get an idea of LESS CSS:
http://gantry-framework.org/documentation/joomla/advanced/less_css.md