The challenge is that the size is different for each layout, because the layouts are technically different. Also, the folder icon size is relative because it must scale within each folder to match the layout size. Anyway, the default size for the folder SVG icon for ROWS and GRID layouts:
.files-grid .files-svg.svg-folder, .files-rows .files-svg.svg-folder {
padding: 0 15% 20px;
}
Keep in mind, when you edit the % for the ROWS layout, it will affect the layout in ways you might not imagine, because this layout involves scaling each item to match the row height, which is affected by the tallest item in the row. I can't really recommend editing this, but if you do, keep in mind you are messing with the layout.
For the other layouts, there is different CSS that sets the size.
revo I thought this was a setting for the size of folders
--menu-folder-icon-size ?
Not quite sure where you found this, but it doesn't affect the large folder SVG icon.