According to http://www.turnpoint.net/wireless/cantennahowto.html beer can would be too small. Optimal diameter should be between 3" and 3.66", however some people reported good results with much larger diameters.
But yes, the opening should be at least 1/2 the wavelength of the signal you seek. The lambda of 2.4GHz is around 0.125m, or 4.9 inches. Half this is just under 2.5 inches.