Definition and Etymology of "Diplasiology"

Reading Benjamin Jowett's translation of the Phaedrus of Plato, I have come across the word diplasiology:

And there is also Polus, who has schools of diplasiology, and gnomology, and eikonology, and who teaches in them the words of which Licymnius made him a present; they were to give a polish.

What does this word mean? What are its constituent Greek roots?

1. Am I correct in assuming that di- is the common binary prefix (from δίς)?
2. Plasi- is the stem I am completely ignorant of.
3. I know of course that -logy is from λόγος.

According to this,

"Diplasiology” means “doubling” words; the Neoplatonic commentator Hermeias understood the term literally, as in “Alas, alas”; modern scholars tend to think words compounded of two roots are referred to.

Sorry I cannot help with the Greek roots, aside from pointing out that your guess on di- looks right, but maybe that definition will help point you to the right stem, too.

This is to complement @JeffSahol's answer. Apparently plasi- is from πλάσις, a molding or formation, which is from πλάσσω, "I mold" or "I form"; this is also the root of plastic and plasma; see the Free Dictionary on -plasia, a suffix used in medical terms such as dysplasia.