There is no single correct size. The standard rig has changed slightly over the years, (e.g. main batten positioning) and some customers, at Jeckells and elsewhere, have also asked for non-standard sizes.
As there is not a formal class association and no official national/international racing specifications, it's always been down to owners make up there own mind about what they want when replacing sails.
I made no special demands of Jeckells when I ordered my replacement suit in 2005, believing there to be a standard size. While the jib probably was the same size (I've never laid it out on top of the old one), the new main is definitely significantly bigger. Some drawings from John Bennett suggest that this may be because later boats were intended to have their booms set lower than on the earliest boats.