This extension allows to downsample all textures in the model or selection. This can be useful if you want to quickly share a file that is too big because of extensive textures, or you used photos directly from the camera and want to batch-downsample them to an optimal resolution.
Download:
https://extensions.sketchup.com/extension/9e33d3fb-6b48-46f9-94b9-0662357e9fee/texture-resizer
Requirements: ImageMagick