RadVirtualGrid exposes an API allowing resizing of its rows. In order to utilize it we need to set the AllowRowResize property to true.
this.radVirtualGrid1.AllowRowResize = true;
Me.RadVirtualGrid1.AllowRowResize = True
The grid rows can be resized by end users simply by positioning the mouse over the horizontal grid line and dragging it to a desired size.
See this topic for more information on how the columns can be resized programmatically.