How can I count Characters and enter its result on the Order page?

I have created a Custom Field on a Product Page, which allows shoppers to enter custom text that they would like to appear on their product. I have managed to Output their Input to the Order Page, however I now wish to be able to create a function which counts the characters they enter and Outputs this on the Order page.

I realise I would need something like echo strlen($inputvariable); but how would I link this to the Custom Field as obviously, it would not be text entered by myself but that of the Shopper.