ENH: improve default scaling in report and improve plot_topomap scaling

closes #1597 

cc @mainakjas @agramfort 
cc @jdsitt
what is the interpretation for `scale`? Isn't it usually such that scale > 1 gives larger than the original size and scale < 1 gives smaller than the original size? Isn't it better to do `myscale = fig.get_figwidth() * 5 * scale` or something of that sort so that we have an easier interpretation?