How can I work out what my background texture dimensions should be?

Newbie & Debugging Questions
#1

I can’t seem to get my background to look smooth and not blurred.

Atm my game is 1280 x 720. The pixels per meter ratio is 1:20. My background texture is 400 x 225. It fits well in the zone however the image looks blurred.

How can I work out what dimension my backgrounds should be based on my pixel per meter ratio and game resolution? This has me really confused :emo:

Here’s what I have so far:

@Override
public void show() {
   ...
   b1 = new Texture(Gdx.files.internal("backgrounds/spr_stars01.png"), true);
   b1.setFilter(TextureFilter.Linear, TextureFilter.Linear);
   ...
}

@Override
public void render(float delta) { //(43, 55, 61)
   ...
   Gdx.gl.glClearColor(132/255f, 0/255f, 35/255f, 1f);
   Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
   world.step(TIMESTEP, VELOCITYITERATIONS, POSITIONITERATIONS);
   world.getBodies(bodies);

   camera.position.set(player.getPosition().x, player.getPosition().y, 0);
   camera.update();

   timer += (delta*1.25f);
   batch.setProjectionMatrix(camera.combined);

   batch.begin();
   batch.draw(b1, player.getPosition().x/20, player.getPosition().y/20);
   batch.end();
   ...
}

@Override
public void resize(int width, int height) {
    camera.viewportWidth = 1200 / 20; //60m
    camera.viewportHeight = 800 / 20; //40m
    hud.getViewport().update(width, height);
}

The result:

The background: