Graphviz, changing the size of edge

I wanted to supplement shuvalov’s answer. penwidth is indeed the correct command. Additionally, in shuvalov’s answer penwidth is both a node and an edge property–also correct.

The distinction i wanted to make:

• penwidth, when used as a node
  property (e.g., “NodeA” [penwidth =
  5]) affects the border line weight
  for that node

• penwidth, when used as a edge
  property affects the line weight of
  the edge (default value is “1”,
  specifying penwidth=2 will make the
  edge appear in bold type

• if you want to change the line weight
  of an edge, you do not need to change
  penwidth for the two nodes
  connected by that edge (as shuvalev’s
  answer might suggest)

• for a directed graph (the edges have
  a direction) you might also wish to change the
  size/weight of the arrowhead and
  arrowtail, along with the edge
  weight, so that all three remain
  proportional

• the length of an edge can be changed
  by setting the weight property, as
  elsewhere, the default value is 1.0;
  increasing that value increases the
  cost of stretching this edge during
  rendering (i.e., the drawing
  algorithm applies a higher penalty to
  solutions in which this edge is
  longer); notice that the edge from 1
  to 4 is shorter than the edge from 1
  to 2.

The following code should illustrate all of this. The rendered graph is shown below the code.

digraph {
    /* declare the node & style them */
    "Node 1" [shape=diamond, penwidth=3, style=filled, fillcolor="#FCD975"];
    "Node 2" [style=filled,fillcolor="#9ACEEB" ];
    "Node 3" [shape=diamond, style=filled, fillcolor="#FCD975" ];
    "Node 4" [style=filled, fillcolor="#9ACEEB" ]

    /* declare the edges & style them */
    "Node 1" -> "Node 2" [dir=none, weight=1, penwidth=3] ;
    "Node 1" -> "Node 3" [dir=none, color="#9ACEEB"] ;
    "Node 1" -> "Node 4" [arrowsize=.5, weight=2.]
}