hmw asked:

Excellent - thank you so much for taking the time to create this plugin! I was just wondering how I can change the font size and/or the size of the image box?

The size of the image box is changed at the top of the plugin file (to edit the plugin file, go to Plugins > Plugin Editor > Peter's Custom Anti-Spam):

// Set the anti-spam image width and height.
// You may need to increase these sizes for longer words and/or bigger fonts.
$cas_imgwidth = 160;
$cas_imgheight = 50;

The size of the font is a bit more buried in the code:

$cas_fontsize = 28; // Default font size for the anti-spam image