What is the optimum ratio for achieving buoyancy using a standard toy balloon filled with urine and helium?

First of all, WHY would you desire to put urine in a balloon? Retaliation? You would need a very large balloon for enough helium to support just 2 ounces of a liquid, be it water, milk, or urine. It is possible, through basic Internet research, to find out the lifting power of a given volume of helium. From there you will be able to determine the size balloon you will need with the formula of a sphere, which gives you the volume of a balloon of any given size. In fact, if you research YA with the right key words (12 inch balloon volume) you will find an answer someone gave. Hope I pointed you in the right direction. You may also want to research attorneys, in case the person you drop urine on decides to file a lawsuit against you and you need to defend yourself against the charge of battery.