What is the best length for a shaft

Have just been reading through mcdood's thread about oscillating hoe's and noticed some comments regarding the length of shafts, as I am doing a lot of shovel work just now, one of the problems I have is the shaft of the shovel being too short, I have to use a shovel which I bought in lidls it has a long shaft but no handle which means you don't get the same power when digging in to lift some soil I have a couple of garden spades and of those the most comfortable one for working with is a cheap one bought again from lidles, as it has a longer shaft, I think it was manufactured in the Continent and it is so much better for working with, when I purchased the other spade which came from an ironmongers, it was the longest shaft I could find at the time, but even on the rare occasion when you come across replacement shafts they are still on the short side, so why the difference between here and the Continent