Most convenient way to set legend marker size?

I always do it like this:

cm = 72/2.54;
BarChart[{{1, 2, 3}, {1, 3, 2}}, 
  ChartLegends -> SwatchLegend[{"a", "b", "c"}, LegendMarkerSize -> 20], ImageSize -> 4 cm]

top: code from your question, bottom: this code

The colors of the legend are inherited from the BarChart object:

BarChart[{{1, 2, 3}, {1, 3, 2}}, ChartStyle -> {Red, Green, Blue}, 
 ChartLegends -> SwatchLegend[{"a", "b", "c"}, LegendMarkerSize -> 20], ImageSize -> 4 cm]

For ChartLegends, although the documentation does not mention any values other than "Row" and "Column" for the option LegendAppearance, it turns out you can also use this option to set the LegendMarkerSize

BarChart[{{1, 2, 3}, {1, 3, 2}}, ChartStyle -> {Red, Green, Blue},
 LegendAppearance -> {LegendMarkerSize -> 20},
 ChartLegends -> {"a", "b", "c"}, ImageSize -> 4 cm]

In fact, you can use any of the options for SwatchLegend as the setting for LegendAppearance:

Row[BarChart[{{1, 2, 3}, {1, 3, 2}}, ChartStyle -> {Red, Green, Blue},
             ChartLegends -> {"a", "b", "c"}, ImageSize -> 4 cm,
             LegendAppearance -> #] & /@ {
                         {LegendLabel -> "legend"},
                         {LabelStyle -> Red},
                         {LegendLayout -> "ReversedColumn", LegendMarkerSize -> 20},
                         {LegendLayout -> "ReversedRow", LegendMarkerSize -> 20, 
                              LegendMarkers -> "Bubble"}}]

For PlotLegends, this approach does not work.