MediaWiki:Common.css

/* CSS placed here will be applied to all skins */
.mw-body-content {
    max-width: 1250px;
}

.mw-panel {
    width: 240px; /* default is ~160px, adjust as needed */
}

/* Adjust content area to not overlap */
#content, #left-navigation, #footer {
    margin-left: 260px; /* must be >= sidebar width */
}

#p-logo a {
    background-size: contain; /* makes the image scale nicely */
    width: 200px;   /* adjust to your new sidebar width */
    height: 100px;  /* tweak height as needed */
}