I have a 120 inch screen in my soundproof theatre.
I am looking to build a very good HTPC but it needs to be small and silent.
advice on specs?
The 2014 generation of NUCs can handle mid to high-level MadVR settings just fine, they just won't do NNEDI, and struggle with interlaced content on some settings. My i5 Haswell NUC can run Lanczos 3 tap on any content I've tried, and can even do Jinc + AR with non-interlaced content. I would expect the next gen NUCs to be slightly better.
For comparison, I run an NVIDIA 660Ti in my living room HTPC, and it can handle Jinc +AR with all content, but still struggles with NNEDI on some content. I have an NVIDIA 970 on my gaming rig and it can handle NNEDI on all content just fine.
So to answer your question, if you want the absolute best MadVR performance, you will not be able to have a "small and silent" HTPC. The Asus Strix 970 is relatively quiet when not under load, and the fans aren't terrible when they spin up, but the card is
enormous. I had trouble fitting it into a full tower case, much less into an HTPC case. A 660Ti will fit in the largest HTPC case that Silverstone makes (but not into the smaller ones), but I couldn't get the 970 into it (although cleverer people than I am may be able to "fit it" with machine tools).
If you want small and quiet you'll need to make at least a small compromise on performance. If I were you I'd look for some screen capture comparisons of Lanczos vs. Jinc vs. NNEDI scaling and see if you think the difference is worth having a great big computer or not. I do a lot of gaming, so I usually have one or two big cards around so the Home Theater use just sort of piggy backs off of that.
If I were only watching movies, I would probably be happy with the integrated graphics on the NUC. YMMV of course, I only have a 60" screen
You should be able to find comparisons over on the MadVR forums and satisfy yourself.