Probably determined by the maximum size of collet that you have available and can hold on the table?
On my Worden, I use ER20, collets in a shop made holder so the limit 13 mm, which suffices for most purposes, for Drills End Mills or Slot Drills. If a Milling cutter has a shank smaller that the cutting edges, then the limit is set by the shank diameter. ( i e, a 16 mm diameter End mill with a 13 mm shank )
Howard