First, the death of anyone due to a Mentos & soda event is a hoax, look it up at Snopes. And they have the wrong reason for the geyser (they quoted but did not listen to a science teacher). The geyser is formed because the outside of the candy is covered with microscopic pits and cracks, called nucleation sites. Has nothing to do with changing surface tension.

My suggestion below is more for Physics and Chemistry. The second answer (Former MN Teacher) is more appropriate because my response is not really a biology project. Except that there are lakes in Africa that periodically explode, suffocating nearby villages. Thought to be the same thing, excessive dissolved CO2 that is suddenly released. Called Limnic eruption. So maybe you can tie in mentos and limnic effects, and it becomes a biology experiement.

http://en.wikipedia.org/wiki/Limnic_eruption

Got one for you. Instead of Mentos, use dry sand. At small amounts it should be proportional to dose, and you can probably use regular 16 oz bottles (use cheap sodas, but fresh). Use multiple teaspoons or some other measure (weight?) as the concentration or dose on the x-axis, and the height of the geyser or the amount of liquid ejected (catch it in a pail) and measured in a cup as the y-axis. The temperature of the soda is also important - room temperature soda is much more explosive than cold because more gas is available (the gas is less soluble at warmer temperatures). That could be a variable - too. Could be as much as double.

Actually if you were making very fine measurements a stormy day (rather than a cloudless day having a high barometric pressure) would also result in more liquid ejected because atmospheric pressure is lower and more gas will be released - I don't know if this difference will be measurable in this experiment (maybe 3% to 6%).

You could become the expert at your science fair.

At some point the ejection volume (nice term to use) will level out because the flow rate will be too high or limited Carbon Dioxide in the soda. Try it on one bottle before you buy a case.

I haven't tried this but it should work and it is scientific if the dose is controlled and the ejection volume measured.

Good Luck.