如何设置图形填充区域?

如何设置图形以尝试填充分配给它的区域?随着节点数量的增加，它的大小只是减小了，但仍保留在一行中，尽管垂直大小(40)允许向下放置.如果删除了rankdir，则它将垂直放置，但也将放置在一行中.

How to set the graph to try to fill the area allocated to it? With an increase in the number of nodes, it simply decreases in size, but also remains in one line, although the vertical size (40) allows placement down. If you remove the rankdir, then it places vertically, but also in one line.

digraph "test_graph"{
  rankdir = LR;
  bgcolor = whitesmoke;
  graph [size = "15, 40"];
  node [shape = circle,
        style = filled,
        margin = 0,
        fontsize = 14,
        color = sandybrown];
  edge [fontsize = 10,
        arrowhead = vee];
  1->2 [label = "R"];
  2->3 [label = "R"];
  3->4 [label = "R"];
  3->5 [label = "B"];
  4->1 [label = "R"];
  5->6 [label = "U"];
  6->7 [label = "U"];
  7->8 [label = "U"];
  7->9 [label = "F"];
  8->5 [label = "U"];
  9->10 [label = "F"];
  10->11 [label = "D"];
  11->12 [label = "D"];
  12->13 [label = "D"];
  13->10 [label = "D"];
  13->14 [label = "L"];
  14->15 [label = "L"];
  15->16 [label = "D"];
  16->17 [label = "D"];
  17->18 [label = "D"];
  17->19 [label = "L"];
  18->15 [label = "D"];
  19->20 [label = "F"];
  20->21 [label = "F"];
  21->22 [label = "F"];
  21->23 [label = "L"];
  22->19 [label = "F"];
  23->24 [label = "L"];
  24->25 [label = "F"];
}