The question of unusually long dha forms also brings to mind the squared-off swords of Assam and neighboring regions. These resemble the Kachin style sword seen in NW Burma and widely copied in other areas, including Thailand.
The Assamese "dha sword" is carried in an open-faced scabbard and the blade is typically more than 30 inches in length.
In the picture below: the top sword (1) is an Assamese "sword dha" (blade length 32 in.); (2) appears to be a transition between the Assamese and Kachin "sword dha" (blade length 26 in.); (3) is an unusual Kachin style "sword dha" that has a lobed guard in the Chinese style (courtesy of Philip Tom; blade length 27.5 in.); and (4) is a curved Kachin style "sword dha" (blade length 25.5 in.).
Each of these is longer than the usual Kachin style "sword dha" with a blade length that averages about 20-22 inches.
And then there is this one from Artzi's site that is truly massive --
massive dha
Ian.