You can set the legend’s maximum width using legend.maxWidth, but it is not possible to set the minimum width.
This, however, would be quite useful to achieve a consistent alignment of pie charts on a single page. The issue can be observed in this fiddle.
With the ability to specify the minimum width in addition to the maximum width, all pie charts could be aligned. Of course, this would also be possible if we were able to specify a fixed width for the legend.