If referring to the Reporting Table/Crosstab, then the height of the row can be set by selecting the row and using the Layout of the properties panel. See the following screenshot for a visual reference.
If referring to the UI for ASP.NET AJAX, then this question should be asked in the AJAX Grid forums.
Please let me know if you need any additional information. Thank you.
Regards,
Eric R | Senior Technical Support Engineer
Progress Telerik