Your answer states you tried setting the Padding property, but my suggestion was to use the Margin property of the LabelsAppearance. Did you actually meant the Margin property or you have tried the Padding instead? You can also try setting smaller FontSize for the labels through the TextStyle property of the labels.
Regarding your question about setting the diameter of the pie chart - the only way to control the size of the RadHtmlChart is to use the Width and Height properties. The actual chart is rendered according to this dimensions (taking into account other elements such as the labels, the legend, etc., not just the actual chart itself), so that all elements should fit in it. Currently, there is no way to set the size of only the pie chart that appears in the PlotArea, but setting the same dimensions (Width and Height) for all the charts in your dashboard would make them appear as having the same diameter. Could try this approach and inform me if this is applicable in your case?
Another approach could be to use a the legend as a place to populate the items names, instead of using the labels. You can fine tune the legend through its position and background properties or through the TextStyle property (e.g. FontSize, FontFamily, Color, etc.). You can also consider implementing a custom legend that is common for all of your charts. This can be a styled div/span element that is aligned on the dashboard. There is an example with KendoUI's BubbleChart
that shows how to create such a custom legend.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed