I think that if a Young female was fed a good diet and in a large tank 8X4+ with good water for all of her life and she had a large bloodline then 30" would be easy and expected.
From what I have read a male is big at 24"
I highly recommend people looking at a 30" Ray face to face as you just cannot comprehend how big the animal looks in a tank, they are really deep (top to bottom) and they make an 8X4 look like a little tank.
One of my female p14 is close to 20inch disk now and when she comes up the front glass she is massive so I know motoro get a lot bigger so I find it hard to believe that many people could home a large adult female motoro of 24inch even
Males do stay a lot smaller my male p14 was always a lot bigger for the 1st year and about 4-6 months older than my females
Don't under estimate the power of a 18inch + ray it friend is massive if you lay out a ruler at 20inch it doesn't look that big but 20inch disk is a different story
My female Motoro grew from 6-inch disk to 26+ inch disk in 18 months...240 gallon tank weekly 30% water changes, FX5 and two big aquaclear outside filters.....she would "stand" at the fromt glass begging for food and the top of her disk was out of the 28-inch high tank....she was absolutely immense...gave her to a breeder cause I felt bad that she had no room!