When a pie chart is rendered with large labels they may be clipped because the pie chart is not sized correctly. When the legend is enabled the pie chart sizes correctly to handle long labels.
The feature will be completed for Q2 2014 SP1. The characters used for the line breaks are "\n". You can see an example in the KendoUI charting widget http://trykendoui.telerik.com/OQiC
You may also want to follow this item http://feedback.telerik.com/Project/108/Feedback/Details/38633-add-the-ability-to-handle-long-labels-and-titles-in-radhtmlchart as it is related.