Considering hexyl doesn't have a manpage, the --help command is the next best thing available to users. A cool feature of hexyl is that sizes and offsets can be specified in hex notation with the 0x prefix. Unfortunately, nothing in the help text appears to mention it.

Note: I'm doing a class assignment regarding creating tests (from scratch) for an open-source command line program, and I chose hexyl for it. You might have a couple more of these on the way, depending on what the assignment brings to light :)