I wouldn't have though a 50kb limit on an ftp server would make the server very useful. If you have a 10mb file, that's 100 files to create, send, receive and verify everything on the receiving end as being what you started out with. That's a lot of JCL, a lot of ftp steps, lots of places to go wrong in writing/maintaining everything, lots of places to loose data/not put it together correctly at the other end (which is presumably out of your control).
If there is a 50kb limit, presumably it is for a reason, like it doesn't want big files... or what?
DFSORT can SPLIT on record counts, in various ways, on ranges of records, but if you want to split on size of data, you'd have to know beforehand how to express that in records.
I suggest you write out (computer or piece of paper) everything you'd need to do (and they at the other end need to do) to do that properly (including verifying that all the correct data arrives, and no extraneous data is included) and then set out how to do that as one shot with a 10MB file and then go to your boss with that.
If someone, somewhere, doesn't change there mind about something then you just have to spend a lot of time ensuring it is done this way, but properly.