Measuring the hexagonal head with a gauge, it's a shade over 25mm (25.3mm) and, somewhat surprisingly in this metric age, spot on 1 inch. I have used an adjustable spanner! It has a central hole but this is round not square so as to fit a ratchet wrench.
By Arnold S. on 14 April 2014
Yes there is a hole in the base so that you can remove the cassette with the axle/lock nut etc in place. If you have Quick Release the hole also allows you to hold the tool in place using the Quick Release/skewer or just the wheel nut otherwise (recommended)!
By Wordsworth on 25 September 2014