Did you try the something like this? 
$graph->set_x_axis_font('/fonts/arial.ttf', 20);
$graph->set_y_axis_font('/fonts/arial.ttf', 20);

[download]
NOTE: untested! UPDATE: I tried the above and it doesn't work at least in Windows even after specifying the font path. However I tried odha57's suggestion in Using GD on MS Windows and this increased the font size. 
    $graph->set_x_axis_font(GD::Font->MediumBold);
    $graph->set_y_axis_font(GD::Font->MediumBold);

[download]
You may have look into GD::Font to be able to specify the exact font size. Good Luck!

In reply to Re: GD::Graph questions by umasuresh
in thread GD::Graph questions by rpike

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