The machine doesn't decide that, your router does. So if the ISP says to use 1492(PPOE DSL), or 1500(PPPoA) then apply it to the router.
You can run a wireshark trace to find what is currently running
Enter a body
We need Know which value of SMTP MTU for Xerox WC7225.
We have some problems with ISP in Portugal and they ask us this value to analyse the communication.
Many thanks in advance