dx9 texture gets its texture from the renderer (dx9). so this is the node which defines the size of the texture… if you want to have manual control over the texture, turn the inlet “autosize backbuffer” off and set “Backbuffer Height” and “Backuffer Witdth” to your desired resolution… note that big resolution can turn down the render speed a lot depending on your graphics card.
im not sure how the backbuffer size is defined when “autosize backbuffer” is on. i think its the next higher “zweierpotenz” to the size of the renderwindow but im not sure…
hope this helps a bit