I was not able to control the width of the chart, but was able to trick it by 1) searching into the future and/or 2) controlling the width of the legend (which can be a fake one if you don't want it to show, since it'll get cutoff)
Please see my answer to this: http://splunk-base.splunk.com/answers/31061/pdf-report-issue-landscape-format-and-no-csv-data/32333 for details.
I also have a similar issue. I'm plotting a timechart with stacked bar graphs for the
search|timechart count(event_type) by event_type
When it renders on my browser it looks fine but the report cuts off the right hand side and some of the bottom (if i add the legend on the bottom). It looks like a result table formats correctly
It resizes to the print window but maybe it's an issue with timechart?
I'm not aware of any direct controls. Perhaps you could show a little more about the kind of data you have, the chart you have, and the problem that occurs in the resulting pdf.