To achieve what you want, you should set the SplitBarSize property of the RadSplitter to the desired size value in pixels. Note, however, that if you set it to a very big value there must be problems with the visual appearance because the image which is set as a background of the splitbar is 5px big and thus you should set your own image with the desired proper size instead.
More relevant information is available in our online help, e.g below:
I hope that my reply is helpful, let me know how it goes.
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