in simplify (propreties) we have the option to reduce every textures size that are higher that 2k, for example, to this same resolution. Is it possible to simplify individually an image texture? or every image texture from a material? via python of course.
you mean independently from the simplify ? just by changing image size ? If you are looking in to this I have a small experience since I did something similar before few years in order to bake textures and export them for webgl (DAE) with the minimum res I could afford for online apps.
there is a scale property in bpy.data.image object, according to this thread it must be relatively easy to implement it, although from a quick search I found out that this is only for new made images created in blender.
Thus for you will need an external module to accomplish it, so take a closer look on the second response which is mentioned also as the solution.