I’m not sure why the units size/scale for the materials is not correct by default but the material has the correct size in its name. For my flooring below, it’s 120 cm. I typically type in 3 or 4’ (feet) since I work in Imperial Units and that fixes it. You can either scale it in V-Ray or, which I find easier, use SketchUp’s material browser.
For applying materials, I recommend making your groups first leaving everything the default material…and when you’re ready to add V-Ray materials, use either the ‘Apply to selection’ or ‘Apply to layer’ options (by right-clicking the material in V-Ray). There can be some problems caused when you have materials applied to both the raw geometry and to the outside of a group at the same time so try keep that in mind.
Once you have a material looking good in your model, you can always sample it with the eye dropper and apply it to another surface at any time.