I am not sure exactly what your problem is. Are you interested in a reaction with the maximum yield products per input masses, a stoichiometric problem or are you asking about a kinetic matter: how to speed the reaction up?
For the stoichiometric problem:
The reaction between citric acid and baking soda is:
C6H8O7 +3NaHCO3===>Na3C6H5O7 + 3H2O+ 3CO2
So to maximize the yield of CO2 gas (since this is the gas produced and I imagine you are interested in the dramatics!) you should use one mole of citric acid and three moles of baking soda.
These mole numbers can be converted to mass using molecular weights. For baking soda its 84 g/mole and for citric acid its 192 g/mol (roughly).
So the ratio you want to use is 3:1 or 252/192 = 1.31. That is for every 1 gram of citric acid use 1.3 g of baking soda. This will give you the maximum amount of CO2.
Then, for example, if you use, say, 25 g of citric then mix that with 32.8 g of baking soda . This will generate 0.39 moles of CO2 or 0.39 moles X 44g/mol = 17.2g of CO2 gas.
You can use the ideal gas law to determine what the VOLUME of CO2 gas will be. After all, the bigger the volume the more dramatic the effect, right?
So P V = n R T and you have P = 105 Pa, T = 298 K, R = 8.314 (SI units), and n = 0.39 moles.
Then V = 0.39 X 8.314 X 298/105 = 0.0097 cubic meters.
Since 1 cubic meter is 106 cm3 we can convert the volume to cubic centimeters a more useful unit. It equals 9663 cm3 that is almost 10,000 ml— a lot of volume expansion.
Finally, as far as the kinetics goes, the reaction rate is pretty fast at room conditions, is it not? The kinetics of the reaction will depend on Temperature but unless you go to extremes it may not be critically important.