Fontsize is a graph attribute (as well as an edge and node attribute). Doxygen generates a dot file, so, e.g.,:
strict digraph {
graph [ bgcolor=lightgray, resolution=128, fontname=Arial, fontcolor=blue,
fontsize=12 ];
node [ fontname=Arial, fontcolor=blue, fontsize=11];
edge [ fontname=Helvetica, fontcolor=red, fontsize=10 ];
}
Specific settings will override generic ones; hence setting fontsize as a node attribute will override the fontsize set as a graph attribute (just for nodes though), and setting fontsize for specific nodes will override fontsize set for all nodes.
If you try what i have above and it does not seem to work, change the fontsize, search your entire dot file for ‘fontsize’ settings, remove them, and re-set fontsize as a node attribute.
Here is the complete graphviz attribute list.