Selecting the proper length of a ferro rod is a bit like selecting the blade length for a knife. You want it long enough that it is comfortable to use, but not so long that it will break. Ferrocerium is a fairly brittle alloy. If you get a ferro rod that is too long and it takes a strike to the middle, it can easily break off. This has happened to me before and can sometimes render the ferro rod useless.
If you find a ferro rod that is supported on the side in any way, that is the way to go. For example, some ferro rods are glued to a block of magnesium on the side. This will support the ferro rod, so it is less likely to break. If you cannot find a supported ferro rod, the ideal length is about three inches not including the handle. This is short enough that it is not prone to breaking and long enough that you can comfortably use it without worrying about bumping your hand.
As a final word of warning, be wary of ferro rods that are built into other devices. I have at least a half dozen different survival gadgets that have a built in ferro rod. These include a survival shovel, a bracelet, a credit card multitool, and several others. Often these multitools have tons of functions, but the ferro rod itself is prone to breaking. They typically are made of softer alloys and are thinner. This allows the company to advertise that it has a ferro rod without spending much money on the addition.
The most important thing with ferro rods is always having one. Chances are you could still start a fire in a pinch even if your ferro rod breaks. However, you are always better off selecting one that you are comfortable using.
Want to learn how to use a Ferro rod properly? Here's an in-depth explanation
Want to get your own Ferro Rod? Here's your best option