Letter-file size.
Tab-to-tab is 12-7/8" at the front and 12-3/4" at the rear.
These are follower plates/blocks that compress the paper files toward the front of the file drawer. Keeps sparse files from scattering toward the rear of the drawer. Keeps all files from spilling their contents when the cabinet is moved or tipped.
Each is a single-piece plate that snugs into the side tracks on the inside of the drawer. The outer tabs have a pin that fits a track with perforations on the lower side (final photo). Also works on those that don't have these perforations, relying only on the tight fit. The tab, including the pin, fits the standard-height track of 1-5/16".