When I import regular OBJ files into Blender, they come in with a basic diffuse shader applied, with the texture file in the right place. Sadly, that texture is often scaled incorrectly. While it is possible to edit the UVs to make it all look handsome, there is an easier way for us to scale textures, namely by adding a Mapping Node into the shader. Here’s how it worked for me.
In the Node Editor, add a Mapping Node (SHIFT+A, then find it under Vector). Leave it on the Point Tab and connect its output to the Vector Input of the Image Texture (the purple one).
Notice that the effect is probably not what you’re looking for. You’ll also need to add a Texture Coordinate Node in front of that, connecting the UV output to the Vector Input of the Mapping Node. I guess otherwise, the Mapping Node doesn’t know what to scale – which makes sense. Once connected, you should see no difference – but no texture weirdness either (other than the wrong scale).
To make a texture smaller, increase all the Scale Values in the Mapping Node. To decrease the texture size, increase the Scale Values on the Mapping Node.
If this inverse law of doing things is freaking you out, switch the Mapping Node over from Point to Texture. Doing so will allow you to decrease the scale values and see a decrease in texture size (and vice versa), but you’ll be dealing with decimal point values. Whatever works for you.
That’s how to do it! Works great in Blender 2.8 and above. Happy fiddling!