You take any instrument or vocal recording you want to "enlarge". Speed it up, doesn't matter by how much, faster will make for a bigger effect. Take that recording and play it out of a good speaker in the preferred room while recording the whole thing with mics at a high sample rate. Stereo pair or even some handheld thing, naturally the higher the quality, better the recording. Then take that recording and slow it down/pitch it down by the same amount you sped up the original sound essentially bringing it back to sounding normal, but the sound of the room will appear larger than reality. I guess because when you slow the last file down it artificially enlarges the perceived space. Really cool effect, although mic placement and the speaker and the room play the bigger role in this, so it takes a bit of getting used to
