Please go to My Site → Customize → CSS (or Additional CSS), then add all of the following code to the bottom of that window.
/* This CSS allows the logo to be made larger.
* 11408945-hc (jw)
*/
header.site-header .custom-logo, header.site-header .retina-logo {
max-width: 300px;
}
Tip: keep the /*
comment */
in the code to later know what this code does.