Speaking from complete ignorance, but after a few minutes of net research ...
Actually, according to duduk fingering charts like
this, the major scale tonic is one or two holes above the bell tone. So, for example, a duduk in A (La) has a bell tone of F#, and would be approximately the size of a high F whistle. A Generation high F whistle measures 10" long. So accounting for different bore etc., an A duduk length of 11" seems reasonable.
The same goes for the other keys and lengths, such as listed here:
http://www.worldmusicalinstruments.com/ ... goryID=104
Roughly, take the key of the duduk, and lower it 2 notes (a major 3rd) to get the corresponding whistle size. The F (Fa) duduk would correspond to a high D whistle, the D (Re) duduk to a Bb whistle, etc.
BTW, the above refers to the Armenian double-reed duduk. I gather that there is also a Balkan fipple flute/whistle which is also called a duduk, and is a different instrument.
Hope that helps.