What is the length field in SHA-512 padding?
In $\text{SHA-512}$ the size of the blocks is 1024 bit. The last block must contain:


the rest of data in message (mod 1024).
some filling (padding)
the last 128 bits as length




If the message is 1919 bit length:


Calculate the size of the data in the last block:
$1919 \mod 1024 = 895$
Add the size of length field(128 bit) to the last block size(895 bit), $128 + 895 = 1023$
See that we have to add 1 bit as padding to the last block to became 1024 bit


The answer will be:


size of padding field = 1 bit 
data of padding field = 1 
data of length field = 1919 as an unsigned 128-bit big endian integer 0x0000000000000000000000000000077F