help(plt.plot) shows
markersize or ms: float
so it appears plt.plot does not allow the markersize to be an array.
You could use plt.scatter however:
ax1.scatter(data[0], data[1], marker="o", c="b", s=data[2], label="the data")
PS. You can also verify that plt.plot‘s markersize must be a float by searching for “markersize” in the official documentation.