Take a look at this thread, you might find it helpful:
Currently, there is no way of specifying the width of each bar. This is automatically calculated taking into account the number of bars, series, and interSeriesPadding and interSeriesSetPadding. If you want to modify the widths, you’ll have to manipulate the inputs to our width calculating formula:
( (minSpacing * (1 - interSeriesSetPadding) ) / numColSeries) * (1 - interSetPadding)
Also, it is important to note, that widths are calculated in terms of data, not pixels.