Source: https://www.elegantthemes.com/blog/tips-tricks/9-useful-divi-css-snippets-you-can-add-to-epanel-in-seconds#3-change-the-sidebar-width

@media only screen and ( min-width: 1200px ) {
.et_right_sidebar #sidebar .et_pb_widget {
margin-right:30px !important;
}
.et_left_sidebar #sidebar .et_pb_widget {
margin-left:30px !important;
}
.et_right_sidebar #left-area,
.et_left_sidebar #left-area {
width:720px !important; /* 1020 - width */
}
.et_right_sidebar #main-content .container:before {
right:300px !important; /* width */
}
.et_left_sidebar #main-content .container:before {
left:300px !important; /* width */
}
.et_right_sidebar #sidebar,
.et_left_sidebar #sidebar {
width:300px !important; /* width */
}
}
Code language: CSS (css)