What size should a room be to create a major chord?

At my school, there are small “practice rooms” adjacent to the main band room. These are small rooms with solid walls, so they create a lot of echo and distort the sound quality of the instruments. Anyways, a few days ago I discovered that one of the rooms seems to be able to resonate at a frequency of concert G below middle C. Whenever that note is produced, the sound of the note is amplified to be much louder than any other note. My best guess for how this occurs is that the wavelength of the note matches the length of the room.

It is well known that rooms also have width and height along with length, which means that it could resonate at many frequencies. Would it be possible to tune the room to amplify a specific major chord (like G C E G)?

What would be the required shape and size for the room?