If you have any room made of stone check it out. Especially if the floor is carpeted. I found as long as I'm a couple feet from a stone wall and the room is pretty dead it sounds great for vocals and acoustic guitar. I think a lot of "warmth" comes with "close sounds" and that's usually best in a dead space, or outside if you know of anywhere absent of substantial traffic or bugs. I don't know if this touches on the subject: "size", but dead spaces I don't think depend much on size, just whether they're dead.