how to change default font size for graphviz?

I use doxygen + graphviz for documenting my code. graphviz does a nice job of generating images.

Is there any way to change the default fontsize for graphviz? The default is 14, but I want to use 12 instead.

Its a real pain to change the fontsize for individual elements like, nodes, subgraphs, edges ... etc.

UPDATE:

For reference here is the code I'm using in doxygen, (the text fields have been renamed, of course)

@dot
 strict digraph {
   node [shape = box, fontsize = 12];
     subgraph cluster_main {
       fontsize = 12;
       shape    = box;
       label    = "main";
       subgraph cluster_main_common {
         fontsize = 12;
         shape    = box;
         label    = "common";
         subgraph cluster_main_common_source {
           fontsize = 12;
           shape    = box;
           label    = "source"
           subgraph cluster_file1 {
             fontsize = 12;
             shape    = box;
             label    = "file1.c";
             gSystem [label = "var1" URL = "\ref var1"];
           }
           subgraph cluster_file2 {
             fontsize = 12;
             shape    = box;
             label    = "file2.c";
             gCard [label = "var2" URL = "\ref var2"];
           }
           subgraph cluster_file3 {
             fontsize = 12;
             shape    = box;
             label    = "file3.c";
             gPwrSupply [label = "var3" URL = "\ref var3"];
           }
         }
       }
       subgraph cluster_main_docs {
         fontsize = 12;
         shape    = box;
         label    = "docs";
         subgraph cluster_main_docs_features {
           fontsize = 12;
           shape    = box;
           label    = "features";
           subgraph cluster_main_docs_features_source {
             fontsize = 12;
             shape    = box;
             label    = "source"
             subgraph cluster_file4 {
               fontsize = 12;
               shape    = box;