OK, I checked the html files in the 1.9.17's dictionaries (back then these were not encrypted but gzipped), and counted the words - actually counted the < b > tags.
The English monolingual has 79667 records. (the MW Dictionary and Thesaurus for Kindle claimed to has 73000 entries)
The English-German has only 16304. (the MW English-German Dictionary has 19000)
Probably these are not the exact word counts but close. And the dictionaries could be improved in the 2.x firmware but the file sizes are almost identical, so I guess these dictionaries still the same.
BTW, the most comprehensive Merriam-Webster (the Collegiate) has 165000 entries.