I don't know if this is the smallest drone, but it is really cute, smaller than your hand, and it has a built-in camera. See this review from Quadcopter 101:
https://www.youtube.com/watch?v=hn926adtGQ4
I just bought two of it, one as a gift and one for me. Flying it feels like the big quadro-copters and is a lot of fun. Indoor is no problem. It has only auto-level mode, no acro mode, but you can setup the speed rate in three steps. With the highest rate, flips and rolls are possible. The lowest rate is good for beginners. I bought it at Amazon for EUR 36.99, just search for "Eachine E10C PRO".
PS: I'm not affiliated with the Youtube channel or the copter company, wanted just share some amazing and fun gadget.