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
You need to have ImageMagick installed. You can get it from the official site (Windows|OS X|Linux). You can install to any location, the extension will ask you initially for the path to the executable.
Usage: (Menu) Plugins → Resize Textures
Without selection, it resizes all materials in the model. If entities are selected you can choose whether you want only the selected faces to be affected (only in selection) or whether the selected materials should be resized in the whole model.
Resizing can be performed either to a given percentage, or optimized to a specific resolution (pixel per meter) or optimized for a specific view (using the plugin Goldilocks if it is installed).
Textures can be cropped to the area that is actually used in the model. If desired, the plugin rotates the crop boundary so that it fits best.