I digged a bit deeper into the matter, and will give a first own answer. Of course people are welcome to give more elaborate answers, the best of which I will accept.
- Origin is the upper left corner of the image
4. Should be the length of the projection in pixels.
Theta is the clockwise angle between the projection and the x-axis (the horizontal axis), which should jump from ~90° to ~-90° when passing the vertical axis. Rho is the length of the projection and is negative, if the projection is "at the left side" of the vertical axis. Therefore, a line passing through (-1,0) and (0,1) roughly has the values rho = -6 and theta = -55°.
I think theta values from -90° to 90° are sufficient, because we only deal with lines passing the first quadrant. An interesting question hence would be how to easily change the origin of the hough space to the center of the image for example.