I think since 'Mantilla' is usually a generic term thrown around these days, much like 'Teacup' for small rays, Mantilla is term for a ray with a sporadic scribble pattern - There is no definitive max size at this point.
Dr. Ross and Jose Mantilla were harvesting and exporting rays out of the wild with this 'Mantilla' pattern - and that is where the name originated. Soon thereafter, many hybrids from Asia would also get this label because of the similar color pattern to what was known in the hobby as 'Mantilla'.. I have also heard that they stay small, in the 14" range, but unless you can definitively prove that the ray was harvest out of the wild from Brazil, it could just be a guessing game when it comes to genetics and max size. Not to mention, there is a good possibility that some rays we call mantilla might be a geographic morph, while other rays we call mantilla could be a product of hybridization in the wild.. because there are definitely some hybrids coming from asia that look identical to the 'mantilla' rays harvest from Brazil.
Sooo.. its possible that the true Mantilla that was harvested from a specific locale by Dr. Ross/Mantilla in the mid-90s may stay small - but at this point Mantilla should just be considered a generic term to describe rays with these type of patterns.