In most of the castle, stuff will be similar to real world human sized things, but very slightly larger. That is to say, instead of 8 feet being the standard ceiling height it would be 10 feet, and so on; there are some fairly large fae so they try to build with those individuals in mind, but the fact is they weren't expecting the Adopted to be this... Differently-sized and shaped, so most everything in their wing of the castle is roughly "human sized".

But please feel free to come up with your own explanation for why Val gets furniture he can actually fit in if you want that for your giant mothman. There could be a bed at the market from an ex-basketball player the fae kidnapped, or he could ask his court ruler for a nice, big easy chair, or he could just make due with the glorified Labubu furniture everyone else seems to be able to fit in. Completely up to you!