MTU option tells what maximum transmission unit is expected to get from TUN/TAP interface. It is per-user configuration. Incoming packets of bigger sizes (including the padding byte) will be ignored. If either noise, encryptionless mode or CPR are enabled, then all outgoing packets are filled up to that MTU value.
Default MTU equals to 1515 bytes (1500 bytes of Ethernet payload, 15 bytes of Ethernet header).