For those who are going on the Masterclass Eugene has sent the following:

Hi Roger,
My fault but all dimensions are in mm.  However, the 15 x 15 section does have to be gripped in a chuck.  My Record/Nova chucks will just grip this but I forgot to consider your chucks.  It might be better to go up to 20 x 20 mm or whatever size is handiest for you.
Eugene