Thanx a lot, it´s first time I see the answer !!

To be more precise, and for those who work in other units than inches like in Europa, let's take an example :

Wood parquet floor in SU : go and check texture size by clicking on Edit in SU Materials.
I work in cm, so it tells me : 304.8 cm x 121.9 cm.

You convert in inches (1 inch = 2.54 cm), or for lazy bums, you go and change units in Model info / units.

So it turns that texture size is 120" x 48".
Take the smaller value, and inverse : 1/48 = 0.02

Fill in Repeat on 3 axis (even though this a 2D image) : X 0.02, Y 0.02 Z 0.02
And that´s it !

@pololo said:

but, it will be awesome to have a shadow matte in vray sketchup. 😄

There are some Shadow channels - you even see one of them in the screenshot. Don't remember what the various do though.