The minimum size of the graft should be 6 mm. The graft must be large enough to substitute for all, or as much as possible, of the conus; otherwise the deformity is liable to advance. When the conus is off center, the graft should be displaced accordingly in order to remove as much as possible of the protruding area.
Reference: Castroviejo R. Keratoplasty in treatment of keratoconus. Arch Ophthalmol. 1950;43:941.
A look at the past . . .. Arch Ophthalmol. 2000;118(7):1000. doi:10-1001/pubs.Ophthalmol.-ISSN-0003-9950-118-7-eya90001