[css-shaders] render size?

How do you specify the resolution of the texture the CSS-shader renderer
elements will be rendered too?

If I apply a CSS-shader to 100x100 pixel element, I can just put

  gl_Positoin = u_projectionMatrix *  a_position * 100.0;

In order to render that element I now need a 10000x10000 texture. At least
as I currently understand it. That element could be 3D CSSed to project into
the background so I don't want to see it's edges cut off. It also needs to
get composited with other elements on the page so I can't just "render to
the backbuffer".