You can change the row height by using Rows.SetNodeHeight method.
But to implement the layout mentioned in your question will be better to use NestedGrids - see the NestedGrids, Virtual Nested Grid sample.
Or you can use SeparatorNode nodes to separate subrows. See the Group ing samples.
[c#]
private AddGroup(NodeCollection nodes, string groupName, object[][] data)
{
NodeBase groupNode = new SeparatorNode(groupName);
Node[] subnodes = new Node[data.Length];
for(int i=0; i < subnodes.Length; i++)
{
subnodes[i] = new Node(data[i]);
}
groupNode.Items.AddRange(subnodes);
nodes.Add(groupNode);
}