I used one of the canisters that the nutrafin co2 ladders come with and it is a little bigger than a water bottle on my 2.5g. It worked fine with a nano glass diffuser. You will have to play around with the mixture though to get the right amount of bubbles. Search for a diy mixture for a nutrafin canister and you should be able to base your mixture off of that.
"For the Hagen Nutrafin Canister:
Sugar to the bottom inside line,
Water to the top inside line,
1/8-1/4 teaspoon yeast
1/4-1 teaspoon baking soda"
-taken from John N.
So probably like a cup to 1 cup and a half of sugar and fill the water to the point where the bottle starts to curve to the lid. I would try to get one of those extra large water bottles though.