I've got a number of ICA slide-in holders, two of which are pictured below. One is stamped 721/6 and the other is stamped 722/6 but both have the same dimensions and fit all of my 9x12 cameras that take this kind of holder. They both measure 10cm wide and 15cm tall (exclusive of the dark slide lip). The film sheaths I have for them are pretty much 9x12cm, or 3.54" x 4.72". Hope this helps. Sorry, but I don't have a Nixe myself. I've been shooting lots of 9x12 lately, though. Fun format and I love these old German folders.

Jonathan