"at" or "in"?

"in" means that you are within the boundary

"at" means that you are either within the boundary, or close by; to be at your house, you don't necessarily have to be inside it.

In many cases both words work and it is a matter of preference

Hoo, boy, this is a hard one. Size may have something to do with it. You probably wouldn't say "at my room", but rather "in my room". "In" conveys, logically, more of the concept of actually being inside something than "at" does. You might say "Let's meet at my house" but not "Let's meet in my house" (unless maybe you were thinking about holding an actual meeting in your living room, say, while you were saying that). If something is bigger, like the airport, you're likely to say "I'll see you at the airport" (not "at airport"), not "in the airport". But on the other hand, if it's really big, and you can't really center yourself on it, you'd say "in". I live in the country. I live in the city. I don't live "at the city".

Rereading that, it didn't clear it up at all. (That's an idiom. You can't say "clear it up in all". However you can say "America has 50 states in all").

Thank you very much,especially to username.

No, you'd probably say " At the door to my room".

"We'll meet at my room before the game."