perhaps i could attempt a rough estimation based on lexicon entries. The biggest dictionary i know is a greek translation and adaptation of Liddel/Scott.
It has 14 volumes, each volume around 600 pages, each page roughly 25 entries. Since the lexicon includes inflected forms etc, let's say 20entries per page. so, 14x20x600=168000 words . So if this method is somehow valid, the greek vocabulary is at least this big..
Some people speak of up to 50 million words in Greek
I dont know about that. perhaps if we count inflected forms - given that a verb has ~500 forms. The only people i know of, that support that 50mill. thing, are extreme-rightwing tv personas that come on tv between gym-o-matic informercials and persian carpet tv-auctions.
A Thesaurus is something else though. Diels said:http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus:text:1999.04.0057
Any one who bears in mind the bulk of Greek literature, which is at least 10 times as great [as that of Latin], its dialectical variations, its incredible wealth of forms, the obstinate persistence of the classical speech for thousands of years down to the fall of Constantinople, or, if you will, until the present day: who knows, moreover, that the editions of almost all the Greek classics are entirely unsuited for the purposes of slipping, that for many important writers no critical editions whatever exist: and who considers the state of our collections of fragments and special Lexica, will see that at the present time all the bases upon which a Greek Thesaurus could be erected are lacking.
But even if we were to assume that we possessed such editions and collections from Homer down to Nonnus, or (as Krumbacher proposed in London) down to Apostolius, and further that they had all been worked over, slipped, or excerpted by a gigantic staff of scholars, and that a great house had preserved and stored the thousands of boxes, whence would come the time, money, and power to sift these millions of slips and to bring Nous into this Chaos ? Since the proportion of Latin to Greek Literature is about 1:10, the office work of the Greek Thesaurus would occupy at least 100 scholars. At their head there would have to be a general editor, who, however, would be more of a general than an editor. And if this editorial cohort were really to perform its task punctually, and if the Association of Academies, which, as is well known, has not a penny of its own, were to raise the ten million marks necessary for the completion of (say) 120 volumes; and if scholars were to become so opulent that they could afford to purchase the Thesaurus Graecus for (say) 6,ooo marks-how could one read and use such a monstrosity?