obstacle circles not consistent when increasing figure size #1396

Describe the bug
in many scripts e.g. PathPlanning/RRTDubins/rrt_dubins.py
the black circles change size when making the figure full screen. This is caused by the fact that the obstacle is drawn with plt.plot(ox, oy, "ok", ms=30 * size), that does not scale the circle properly when the figure changes size.
probably something like

ax = plt.gca()
ax.add_patch(plt.Circle((ox, oy), size, color="k", fill=True))

would work