Make default `figsize` default argument valid for both backends.

[juanitorduz]

After adding matplotlib support we left the large figsize plot for the lets-plot backend. Hence, using the default value in the matplotlib backend creates huge plots that will kill the kernel. We should make this default value work with both backends.

Idea: Something like (plesudo code)

def my_plot(figsize=None, plot_backend="lets-plot"):
    if figsize is None and plot_backend=="lets-plot":
        figsize = (100, 100)
    if figsize is None and plot_backend=="matplotlib":
        figsize = (10, 10)
    ...