You don't have to configure anything.
If there is a too long low pulse on receive wire, so that the stop bit is not idle, you get a framing error flag in the status register for the received byte.
So if it is a real break condition, you will receive a 0x00 byte, and then you need to check if this byte has a framing error.
It can't really measure how long the break condition was, so if you really need to measure it, you need to do it separately.
But for DMX512, detecting the framing error should be sufficent. DMX512 is not 8N1 but 8N2 framing, and AVR UART will only check the first stop bit.
If you want to check both stop bits, you could always use 9N1 framing on AVR and check the 9th bit yourself.
If you only want to receive with an AVR, because it does not require and check the second stop bit, you can still use 8N1 for reception, the receiving logic does not care if data on wire is 8N1 or 8N2, as for the receiver logic, there's just always one idle bit before start bit. But the transmitter logic has to generate 1 or 2 stop bits before starting next frame.