How to resize the sidebar with CSS

Paste this code in the "Customizations" window:

.sidebar .tree-container {
  width: 400px !important;
}
.page-container_sidebar-open {
  margin-left: 400px;
}

Adjust the "px" values to your liking.

*Note that the changes will only show on the public page, and not in the editor. In the editor you can resize the sidebar (for your device only) simply by dragging its border.