scaling textures for vray is done with the native texture scaling in SU. just right click the face with your material, choose 'texture' and then scale away. or better yet, in the SU material dialogue, adjust the exact measurements of the jpg image.
you can change the displacement parameters in the material to determine grass 'length.'
i suggest searching for related questions at http://forum.asgvis.com as the topic has been covered numerous times and in more depth than i have here. that's the best place to get answers to your vray related questions.