From the swarmplot docstring:
Signature: seaborn.swarmplot(x=None, y=None, hue=None, data=None,
order=None, hue_order=None, split=False, orient=None, color=None,
palette=None, size=5, edgecolor='gray', linewidth=0, ax=None,
**kwargs)
Docstring: Draw a categorical scatterplot with non-overlapping points.
[snip]
Parameters
----------
x, y, hue : names of variables in ``data`` or vector data, optional
Inputs for plotting long-form data. See examples for interpretation.
data : DataFrame, array, or list of arrays, optional
Dataset for plotting. If ``x`` and ``y`` are absent, this is
interpreted as wide-form. Otherwise it is expected to be long-form. [snip]
And then:
size : float, optional
Diameter of the markers, in points. (Although ``plt.scatter`` is used
to draw the points, the ``size`` argument here takes a "normal"
markersize and not size^2 like ``plt.scatter``.