There is a member on here that is involved with the Shades of the Harz Layout that had Wernigerode Box on it. Perhaps he could help?
On a different matter it is not what we would regard as a signal box these days, more like a ROC Railway Operations Centre featuring Controllers, Signalling and possibly even Drivers and perhaps other Staff Accommodations.
It is a big building in G Scale. However if all else fails why not scale up from the pictures, doors 6’6” 3‘ wide will get you a long way to a pretty accurate model. Or like me with my Alexisbad Station Building, selective missing out gives a building that captures the features but takes up less space whilst keeping the believability of the place.