Direct Hot Water Cylinders

I am about to install a new domestic direct hot water cylinder. I would like to find out whether the size of tank bears any relation to the amount of hot water contained. The element in the new tank enters fron the top and I have been inforned that the amount of water heated depends on the length of the element and that the water below the tip of the element remains unheated. Is this correct? Are there any direct hot water cylinders where the element is fitted fron the base, thus heating the whole tank of water. Many thanks in advance. Martin