In FIX what determines a message length? Because I have read if a message exceeds its length it will be sent in fragments.
Splitting up large messages is not an implicit part of the FIX protocol.
Some counterparties may choose to split up data into multiple messages instead of sending giant messages, but they don't have to. In my experience, I've seen counterparties send ridiculously large messages.
If a message is split up, it's because the sending party chose to implement their system that way.