Hi Rashad,
First of all, specify your favourite image editor in your preferences window (Window menu on a PC) see below - I use Gimp to fine tune my images
Application.jpg
Then when you want to edit an image used as texture on a face, right click > Texture > Edit texture image. This will open the image in your editor where you can resample and compress it to your needs and upon saving, SU will instantly refresh the texture while keeping its UV coordinates. Really handy tool introduced in SU 7. Note however that the Texture menu item in the context menu is only available if you right click on a single face - i.e. not on curved surfaces or groups/components.
Scaling components (or anything) bearing textures will not affect the file size.
The model infor > Statistics > Purge will purge everything "purgeable" i.e. (unused) components, materials, layers, styles (each of which can be purged from the respective dialogs/browsers individually, too).
At the moment, you can only do this by right clicking a component (either in the model or in the Component browser) and "Save as..." It will save it as a separate skp file (which it is actually, only embedded into your model) and in your Explorer window you can see the file size.
A good method therefore is to download the different components from the Warehouse in your internet browser, check their effective modelling (file size, polygon count etc.) and only then import them into your model.
It would be cool to see the file size in the component browser especially bnow as the online Warehouse is so tightly integrated - hopefully once it will be implemented.
I think that's it for now.