Get the same gas tank, put the exact amount of gas in gallons that your receipt showed that went into your "2.5" gallon can.

Now go to Walmart and buy some gallon jugs of water and I guarantee you it'll be close.

I have a 5gallon can for our lawnmower and I've topped it off right at 6.4 gallons before. This is simply just because of the can's shape.