LoRAWAN FOTA using chirpstack application server GUI


I am trying to use Chirp stack application server GUI interface to schedule firmware update over air to my end node.

I will like to know and understand how fragmentation of binary file into fragments as per LoRaWAN Fragmented data Block Transport Spec takes place inherently and what is the size of each fragmentation block by default?

And on what basis multicast timeout value needs to be specified in GUI tab? Is it entirely based on file size/ number of fragments along with duty cycle limitations for Rx channel use and data rate/SF settings used. Any resources/help will be greatly appreciated.

Thanks in advance.