When painting textures, regardless of the brush shape the texture only applies to the cross points on the grid (where two yellow lines meet), and then fade out from there. So in reality the shape of the brush doesn't really matter, it's just a visual indication of where the texture is going.
You can also do "square" brushes by opening the Advanced drop-down in the Ground Texture tab. There you can select an area and fill that area with the selected texture. The texture will bleed out from the selected area, as you select along the same grid lines the texture applies to.
peter