From whatever source, scales may be purchased in various lengths to fit different applications. The manufacturer/seller will tell you how to measure for that brand of scales. It is very important that the length of travel is long enough so the read head never can crash at the ends of its travel. If the scale travel is shorter than the axis travel, there is an accident waiting to happen. Scales with a longer travel than the axis travel do not cause a problem unless there are physical constraints to the mounting areas on the machine, other stuff getting in the way. Also, if you have an axis travel of 20" and buy a scale with 20.1" travel, it will work, but you will play hell mounting it accurately enough to not crash at one end or another, and it is difficult to mount to those tolerances. Give yourself an extra inch or two minimum when there are no other mounting constraints, it will make life much easier when mounting the scales. I ordered the scale travels for my mill 50mm longer than the maximum travel of the axes.
One more thing. Do not use the manufacturer's published numbers for travel when ordering scales. Measure the travel carefully yourself, making sure that travel stops and other temporary restraints are not limiting the maximum travel of the axes.