I don't see any sizing indicated for the Nideggen paper, but a sample of it does feel somewhat stiff in the hand, so it doesn't seem to be waterleaf / unsized. Can you let me know what the sizing is? I'm trying to determine if it would take letterpress as well as an unsized sheet or an internally sized sheet such as Rives BFK

The Nideggen paper is not a waterleaf paper and is neutrally sized. It takes letterpress printing very well. Here is some more information about all the Zerkall papers: http://www.zerkall.com/grafik/downloads/Printingpapers.pdf