Hello Bernhard Huber

Sorry, we do not provide customization support. You need to use Custom CSS for it. However, I will give you a hint. Please use this CSS to Template Options -> Custom Code -> Custom CSS. It will make the sidebar of 300px width.

body.predefined-header1 #sp-header {width: 300px; padding: 30px 20px;}
@media screen and (max-width: 1920px) {
body.predefined-header1 #sp-main-body, 
body.predefined-header1 #sp-bottom,
body.predefined-header1 #sp-footer {margin-left: 30px;} 
} 

@media screen and (max-width: 1440px) {
body.predefined-header1 #sp-main-body, 
body.predefined-header1 #sp-bottom,
body.predefined-header1 #sp-footer {margin-left: 10%;} 
} 

@media screen and (max-width: 992px) {
body.predefined-header1 #sp-header {width: 100%;padding: 20px;}
body.predefined-header1 #sp-main-body, 
body.predefined-header1 #sp-bottom, 
body.predefined-header1 #sp-footer {margin-left:0;}
} 

body.predefined-header1 #sp-header .sp-megamenu-wrapper .sp-megamenu-parent > li > .sp-dropdown {
    padding-left: 20px;
}

Best regards