I think 60cm is about the average adult female for miliaris, but the problem is that the literature lists the species getting to a meter or more.
I think part of the problem is that there is confusion about the separation of miliaris and tataricus across their range.

Furthermore, I suspect most miliaris in captivity come from a relatively small area of the species range. Looking at other better known Erycines (Charina/Lichanura) there is some significant variation in maximum sizes across the species range. I suspect the same could be true of miliaris.

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Chris Harrison
San Antonio, Texas