i think for this case we need to split the records one line each by using SplitRecord/SplitText processor.
Then Using MergeContent processor we can do 500 MB splits by using this way we are not going to have splitting records in between.
1.SplitRecord/SplitText //split the flowfile 1 line each
2.MergeRecord/MergeContent //to get 500MB filesize
To force merge flowfiles use MaxBigAge property like 30 mins..etc.
In case if you are using Record oriented processors we need to define Record Writer/Reader with avro schema to read/write the flowfile.
Refer to this link for more details regards to merge content processor.