Can 中型 be used to describe the size of a room or building?
The kanji 型 ("type") implies there is some objective criterion that determines whether something is 小型, 中型, or 大型. Saying 中型の部屋 or 中型の建造物 according to some criterion/law is okay. For example, 中型自動車 has a strict definition in Japanese law.
If you want to subjectively say something is middle-sized, it's better to say 中くらいの大きさの～, 普通の大きさの～, 大きくも小さくもない～, etc.
A definition: 形が大きくもなく小さくもないこと。また、そのもの。 It seems broad enough a definition to apply to anything with a size and other sizes to compare it to. The problem is whether the speaker and listener have a common frame of reference. For cars, it is easy and pre-defined. For rooms, it would seem to depend on your context.
For me as an engineer, objects are relatively large and if they can be classified into three categories by their size as small, medium and large. In addition we can share the classification as the standard categories, we say "中型ＸＸ."
We don't say "小型コップ," "中型コップ," and "大型コップ" in a coffee shop, because cups are not relatively large.
Some people don't say "中型の部屋" because "型(type)" is often used for standardized products or objects, and we can share the classification as the standard categories. Room size can continuously distribute, so "1K," "2DK," and "3LDK" is commonly used.