There is a yet to be named Platystele viewable at this site: http://news.nationalgeographic.com/news/2009/12/091203-worlds-smallest-orchid-picture.html Flowers about 2mm. Of the named or 'real orchids', Platystele jugngermanioides probably takes the cake, it stands 5mm tall and the flowers measure 0.5mm.

I grow Lepanthes chorista which has 3mm flowers but many of them. When it had a flush about a month ago it sported over 50 flowers on the under surface of about 20 leaves. Quite a few of the other Lepanthes have flowers that small. Some are not only small but nondescript and unattractive, the L chorista on the other hand is colorful and very attractive even when viewed without a magnifying glass. Check it out: http://www.orchidspecies.com/lepchorista.htm

Nick