I am planning a small project with BT which must be connected with Android app.
what parts do you advise to buy??
Get serious. You must realize we need more information about your project than what you have provided. If you ask a direct question like what is the smallest arduino available the answer would be a Micro, Nano, or Pro-Mini. Some things can be done with
an 8-pin ATtiny85 ic. (google it for datasheet) It has multiple programable I/O ( analog , PWM & digital.). Pro-Mini's can be found for $3