Salesforce allows for a large number of custom fields. However, they have a maximum URL length when calling their API. The Salesforce element, by default queries against all available fields in each object. For example, when calling the standard GET /contacts URL the element, by default, queries against all available (standard and custom) fields in the contacts object. This can lead to the URL becoming quite long and reach the limit for the request URL. If your response URL is longer than 16,088 characters, you will get a 'Request Header Fields Too Large StatusCode=431'. If it reaches 16,410 characters, you will get 'URI Too Long StatusCode=414'.
If you have a large number of custom fields, you should use the GET /query resource and narrow down which fields are being returned.. For example: