Sorry for not adding more details, let me explain a little further. I am planning on building a barebones arduino(uno)/circuit. I had not thought much about the 5v regulator, but I will include that in my circuit to eliminate any concerns with overvoltage. I am developing my code and setup with a full UNO, but I want to keep the final build small, hence the new minimal circuit.
I can draw up a rough circuit later, but assuming that I include the regulator, what is simple compact option for a battery?