BUG: grouped hist and scatter use old figsize default
Grouped `hist` and `scatter` plots use `figsize` default which is different from pandas current and mpl.

@TomAugspurger ?
 @sinhrks what's the default figure size after this PR?
 regardless, this is ok, just rebase and i'll put this in
 @cpcloud Default will be either the mpl default, or pandas default (`pd.options.display.mpl_style`).

I rebased.
 @cpcloud ?
