With AnyChart, the bar width is something that is calculated automatically. You'll see this mentioned in the AnyChart online documentation on Bar Charts. You could try playing around with the padding options, to see if such settings help.
I've found simple trick to set bar width.
You need to use FLASH version of Anychart and enable ZOOM of AXIS in XML definition.
<zoom enabled="true" show_scroll_bar="true" scroll_bar_position="outside" start="0" visible_range="40" />
You can also set visible_range dynamically. If you set for example 40, you will have 40 values X or Y axis and bars will have same width.