In short: There is no node for this within the material editor
The node system is for JIT operations, and if you want to rescale an image, you probably don’t want to do it everytime you run the nodetree.
But there are work arounds
Blender let’s you change the image instance resolution (instance here means that without saving it to a new file, the rescaled image will be lost when you close the session). But as far as I can tell you have to use the python script panel.
You'd probably have to use something like the
blenderpy code below:
import blenderpy as bpy
width = 2000
height = 2000
bpy.data.images['Directory to Your Image'].scale(width, height)
Or there is a method with
from PIL import Image
basewidth = 300
img = Image.open('fullsized_image.jpg')
wpercent = (basewidth / float(img.size))
hsize = int((float(img.size) * float(wpercent)))
img = img.resize((basewidth, hsize), Image.ANTIALIAS)
Alternatively blender does have an image editor that can adjust resolution and output a new file with the resolution for you to open in the node editor, but I think this option is a little redundant. You may as well resize the image in a purpose built image editor.