Bug introduced in 10.0.2 or ealier and persisting through 11.0.1 or later
Is it possible to change the aspect ratio of the BarLegend in Mathematica?
The option LegendMarkerSize, in my case below, only changes the overall length of the bar.
So, for the example image below, how can I make the colour bar wider? Note that because of the contours argument, using LegendMarkerSize -> {300, 300} has no effect.
BarLegend[{"Rainbow", {0.0, 1.0}}, 20, LegendMarkerSize -> 300,
LegendLabel -> Style["Intensity", Bold, FontSize -> 14],
LabelingFunction -> (Style[NumberForm[#, {Infinity, 1}],
FontSize -> 12] &)]


