Rhaseltine is talking about resizing the textures to a smaller size so that they require less memory to use (but won't have as good appearance at very close range). i.e if a texture is 2048x2048 make it 1024x1024 and use the smaller image.
He is advising that you make a backup copy of the original, full size, textures images so that if you want to use them at full size, you will have them. I'd second that advice.
You could use any image editor, Paint Shop Pro, Photoshop, GIMP, etc.  Anything really should be able to simply resize.

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