Well, these folks are building a new one, not dealing with one that's already there. Tell my characters how big to make it.
This is just a small neighborhood biz, not a major concert venue, so we're talking your standard drums-bass-guitar-singer and maybe an optional keyboard, with some room for them to walk around and emote. But not much. We're not hosting a rock opera here, don't need tympani, xylophones or pyrotechnics.
Just a bit of space to set them apart from their audience. There's not even going to be a dance floor. I know, what a bummer.