iis 7 log file directory
You can get the site id by clicking on the Sites node (major heading). Each site has a site id beside it in the 2nd column. The log folder will be in the format of "w3svc{siteId}", matching the site ID.
You can also find it by going to the site first and clicking on "Advanced Settings..." in the Action pane. Then the ID will be shown as one of the top fields in that dialog box.