I like to buy the 3-foot lengths and then cut them in half for an 18 inch bar .... sink 8 in the handle, leave 10 exposed ... then work the tip and drill/tap it for your carbide tip.
Why so much metal???? Well .... it's all about the smoothness of the cut, actually. The larger and heavier your tool, the smoother it will make the cut, with less chattering and better control over the tool.
There is, of course, some limits to remember ... the heavier the tool is, the more stamina it will take to just hold it up where it needs to be. The heavier and longer the handle is, the more unwieldy it becomes, and making inside cuts on the end of your stock can get pretty difficult if your tool handle is right up against your tailstock.