I can understand that - no problem, as I have "heard" of your lower back issue before. However, for most people a shorter bar vs. bending is not a real issue.
If most of us were short that might be true, but people come in different sizes and shapes and taller people bend up to several inches more. If your cutting firewood a few hours or more maybe a few hundred bends or more, + the amount of time in the bent position while the saw is cutting and the weight of the saw pulling on a bent body.Sounds like good exercise for a few minutes. A bar that reaches ground level while standing straight up using a sharp chain and back pain from cutting firewood is not usually an issue here, an 18" bar and less then perfectly sharp chain is a pain for my back in about 10 minutes usually which I much rather do without and it really sucks when you know it can easily be avoided with a longer bar and properly sharpened chain. Some circumstances are different but most of my cutting goes that way.