The only way is to use a scenery object of the size you need, or a spline of the width you need, rather than applying ground textures. These are very simple to make using, for example, GMax - I made some of these some time ago when I needed some clearly defined areas. They have to float slightly above ground level and a simple two-poly plane is quite sufficient.
If you would like to see an example, look for my KUID2:275817:60676:1 on the DLS. The description says it all:
"A 10 metre by 10 metre square of grey brick paving in herringbone pattern for use when clean edges are needed. This can be used to create an area of any size (in multiples of 10 metres) with right-angled corners or to conceal digholes. 2 polys."
Ray
