I’ve sent you a email, have you receivedThe dimensions for all Robust toolrests are shown here: http://www.turnrobust.com/toolrest-dimensions/
From Steve's description and dimensions, it looks like this might be what Robust calls their "Standard" post length (for 16 inch lathes) rather than their "Short" post length (for 12" lathes).
Robust has another document to help you decide which of their toolrests will fit your lathe, here: http://www.turnrobust.com/robust-toolrest-selection/