Pass a variable into AWS Shadow

Question

I am just learning about how to use shadows in AWS. So I want to be able to pass a variable into AWS so it's read by the Shadow (sorry if I'm not explaining this correctly, but I hope it makes sense). So basically, I have the following code:

Payload = """{ "state":
    { "report":
        {"seconds" : "10",
        "size" : "20"
        }
     },
"message: "Hello World"
}"""

And I get the exact same output in the "Device Shadow state" in AWS. So what I want to do is have seconds and size as a variable.

I have tried setting a variable as:

size = "20"

and then using it as:

Payload = """{ "state":
        { "report":
            {"seconds" : "10",
            "size" : size
            }
         },
    "message: "Hello World"
    }"""

however, this didn't work and the Classic Shadow in AWS disappeared. Can anyone please help?

Thank you!