Make render window size adjustable

When working on my laptop, the default size of the render window did not fit on my screen. Then, when manually dragging the window smaller, the aspect ratio got distorted.

This commit fixes both:

• via the command line argument the figure fits on my screen: --figsize=8, default stays at 10.
• when resizing the window, the aspect of the actual plot stays square. (ax.set_aspect("equal"))