The size or gallon capacity of your trash receptacle is usually printed on the container itself. If not, here are simple instructions for measuring for the correct size. Remember, you should have about 3" to 4" of overhang on the outside of the receptacle.

For a round receptacle:
Width = diameter of receptacle x 3.14 / 2 (diameter is the distance across the top of the receptacle)
Length = 1/2 the diameter + height of the receptacles + 3 or 4 inches for overhang

For a square or rectangular receptacle:
Width = length of two sides
Length = 1/2 the smallest side + height of the receptacle + 3 or 4 inches for overhang