http://www.brewershardware.com/how_to_use_tri_clover_fittings.html
The pipe or valve size doesn't necessary match a tri-clover size. In theory you could have a 3/4 pipe with a 6" tri-clover. That said the 1.984" flange (often called 1" or 1.5") is the most common in homebrewing. Sounds like you...