How can I get String length of my form input fields?

Is there a way of determining string length?  

i.e. I would like to use substring to get me the last 3 digits of a string but I don't know the (variable) length of that input

You can fetch the field names and of your fields as shown in the guide below.

https://www.jotform.com/help/146-How-to-Find-Field-IDs-and-Names With above data you can use Post to a different platform which you can then get the string lenghts of your fields

https://www.jotform.com/help/71-Prepopulating-Fields-to-Your-JotForm-via-URL-Parameters

https://www.jotform.com/help/351-How-to-Automatically-Pass-Form-Data-to-Another-Form

The other option you can also get the source code of your form and use the source code to get details of your form fields. Below is guide of getting source code.

https://www.jotform.com/help/104-How-to-get-the-Full-Source-Code-of-your-Form

Kindly do let us know if you need further assistance.