Claire,

According to http://en.wikipedia.org/wiki/Carbon_dioxide, the density of Carbon Dioxide (CO₂) is 1.98 g/L as a gas and 1600 g/L as a solid (dry ice).

To calculate the volume of a box containing a particular mass of CO₂, you divide the mass you want by the density.

So if I want 200 Kg of CO₂ in dry ice form, then I have 200000/1600 = 125 litres. That would be the volume (box size). The dimensions of a box (length, width and height in centimeters) can be any three positive numbers that multiply to give 125 000 since 150 litres is 125 000 cubic centimeters.

If you are not working in metric, you have to do a lot of icky conversions. When I am faced with that, I just use the unit converter on Google by typing in "1600 grams per liter to tons per cubic foot" and it responds with
"1 600 (grams per liter) = 0.0499423685 short tons per (cubic foot)"

So 1 ton / 0.0499423685 tons per cubic foot) = 20 cubic feet. A twenty cubic foot box would hold this much dry ice.

Cheers,
Stephen La Rocque.