The RTF support for the RadRichTextBox is being developed according to RTF v.1.9.1 specification (you can download it from here).
As for specifying cell widths - your approach by using the \clftsWidth and \clwWidth control words is correct. However the \cellx control word cannot be omitted as it marks the end of cell definition. Also note that according to the RTF the specification cellx defines the right boundary of a table cell. So it is not only the width of the individual cell but the sum of that with and the widths of all the cells before it. In the case when an explicit cell width is specified (using the \clftsWidth and \clwWidth control words) the actual values of the \cellx tags are ignored, but still these values must be in increasing order otherwise you will probably get the "ColumnSpan must be at least 1" exception.
To make thing clearer here is a short example of a table with two columns 40% and 60% of the width:
\pard\intbl\itap1 first cell\cell\pard\intbl\itap1 second cell\cell\trowd\irow0\trgaph0\clftsWidth2\clwWidth2000\cellx2000\clftsWidth2\clwWidth3000\cellx5000\row
Please don't hesitate to contact us again if you have more questions.
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