Another option for you is to set a custom size for a child GridViewInfo programmatically. In order to do so, subscribe to the ChildViewExpanded event and set the custom size by setting the Height property of the ChildRow to a specific value. Please note that the following code will work only when the UseScrollbarsInHierarchy property is set to true.

Copy[C#] Setting a custom size for a child GridViewInfo by passing a specific value

void radGridView1_ChildViewExpanded(object sender, ChildViewExpandedEventArgs e)
{
    e.ChildRow.Height = 300;
}

Copy[VB.NET] Setting a custom size for a child GridViewInfo by passing a specific value

Private Sub RadGridView1_ChildViewExpanded(ByVal sender As Object, ByVal e As Telerik.WinControls.UI.ChildViewExpandedEventArgs) Handles RadGridView1.ChildViewExpanded
    e.ChildRow.Height = 300
End Sub