The constant is called
MAX_BLOCKFILE_SIZE and is set in
src/validation.h. It is currently set to
0x8000000 which is 128 MiB (134,217,728 bytes).
You can see where it is checked in
There isn't any indication of why this specific size was chosen and it may be arbitrary. It is desirable to have some limit because some operating systems (or specific filesystems) have limits on maximum file size, usually above 2 GB or so. So at least this limit ensures we never get anywhere close to an OS limit.