I checked on the Record website and the manual for the DML 320 is strangely missing and so is that information on the website. From the photo it looks quite large - I'd say 25mm or 1" but you really need to know which.
I think your best chance would be to email them or call them (01246 571 020) to find out.
It could be useful to compile a list of post diameters and lengths for different lathes - it could aid buying second hand, from evilbay or even new from different makers. Anyone any thoughts?