Payload calculation

Hi everyone.

I interest about LoraWan. I beginner in this field.
How can i calculate a payload size?
Can anyone give a little hint about it? I found the airtime calculator page.
For example I found this product and manual.Senstick Microclimate SMC30
Can anyone give an simple example that how can I calculate it?
Can I find a manual about this thing?

Thanks a lot for helping