"The maximum uri length of 2,083 characters was exceeded" exception is thrown when you filter RadGridView. Most browsers have a maximum uniform resource locator (URL) length of 2,083 characters. When you filter RadGridView and add a large number of distinct filters - they are added in the URL of the request and when the length is exceeded the exception is thrown.
Solution: Set the ShowDistinctFilter property of the grid column to False to hide the distinct values listbox. The user will have to type in the field filter, i.e. the lower part of the filtering control with the combos.
Another solution would be to stop the built-in filtering of RadGridView (IsFilteringAllowed="False") and use RadDataFilter instead.
In paged RadGridView, the distinct values listbox shows the values from the current page only. The current version WCF RIA Services do not support projections on the server, i.e. you can't make a Select(person=>person.Name).Distinct() on the server out-of-the-box. But this can easily be done with an additional query for every Type of column and the help of a simple dummy class called DistinctValue. A demo showing the approach can be observed here.