What size of flask should you use? Consider the following factors then give your answer:The total volume of liquids that you will mix.The shape of a conical flask - consider that:A rapidly-effervescing (or boiling, or foamy) solution can easily overflow if it rises up the inside of the flask. Will H2O2 release oxygen gas?Stirring will be ineffective if the flask is under-full or over-full. The ideal liquid level is generally not much higher than where the flask begins to taper inwards.Cooling will be more effective if the flask can be immersed in an ice bath to at least the level of the solution. The typical practice to not fill a flask more than 25-50% full (aim for 1/3rd)That only certain flask sizes are availableThat stirring will be ineffective if the flask is over-full or under-full.

To determine the size of the flask you should use, you need to consider several factors:

1. Total Volume of Liquids: You need to know the total volume of the liquids you will be mixing. This will be the minimum size of the flask you need.

2. Shape of the Flask: A conical flask is ideal for rapidly effervescing or boiling solutions as it prevents overflow. If you're using H2O2, it will release oxygen gas which can cause the solution to foam and potentially overflow.

3. Stirring: The flask should not be under-full or over-full to ensure effective stirring. The ideal liquid level is generally not much higher than where the flask begins to taper inwards.

4. Cooling: If you need to cool the solution, the flask should be able to be immersed in an ice bath to at least the level of the solution.

5. Flask Filling: The typical practice is to not fill a flask more than 25-50% full. Aim for 1/3rd full to prevent overflow and ensure effective stirring.

6. Available Flask Sizes: You also need to consider the sizes of flasks that are available to you.

Taking all these factors into consideration, you should be able to determine the appropriate size of the flask for your needs.