Do you know how many characters you're getting in the response?
Also, are you sure the response is being cut off when you run the workflow? Something to note is that the token values shown in the instance history can be truncated.
Before saying for sure that the response was cut off, I would suggest you get a sample API call with a predictable result, then use the Read JSON activity and try to check for a value near the end of the expected response.
It's more likely that things are fine within the workflow instance and it only gets cut off in the places you're looking during testing.
See the following post: https://answers.laserfiche.com/questions/203050/Workflow-HTTP-Web-Request-max-content-length