The image I have under Explorer is 42x9, but is 32x8 when I view it in Unity (and if I get it from script. What do I change to fix this?
In the Inspector, switch “Texture Type” to advanced. Then you will see the option “Non Power of 2”, which has probably defaulted to “ToNearest”. Unity likes to work with textures of power of 2 sizes, so your image will either be scaled down to 32 or up to 64, unless you tell it not to.
This answer gives a good explanation of why you should use textures that are a power of 2.