just found an original - i'll get this one shall I?.. around 1990/1991 the accepted Ducati clutch slave size settled on seemed to be 26 mm so anything of larger diameter than this will make the action at the lever lighter but bear in mind that this comes at the expense of a reduction in travel as regards clutch pressure plate and so will affect gear change quality/feel and particularly when trying to find neutral if too great a change in dia is made. Some are happy with 28 - others prefer 30. I think Oberon standard is 30 now but 29 was available once. Other factors can also make a difference i.e. - if you reduce the size of the master cylinder piston it will have a similar effect or change the lever size and/or it's ratio between pivot and pushrod shackle pin pivot. Best and most enjoyable mod is to remove two springs from clutch assy as said although clutch slip can still result with some combinations.
Cheers Christoff, the Oberon I bought was advertised as 29mm, not measured it. There are some Chinese cheapo ones popping up on eblag for ~£40, I was going to give one a punt, but the 31mm piston size put me off, as Chris rightly says, this can lead to issues with stack separation, leading in turn to probs finding neutral. I did once dabble with four springs, but mine slipped under hard acceleration, so I abandoned it. Guess you need good plates.
