alt Hacker NewsWell, it also has the advantage of providing pretty decent encryption for free through WSS.
But yeah, where that's unnecessary, it's probably just as easy to have a 4-byte length prefix, since TCP handles the checksum and retransmit and everything for you.
It's just a standard TLS layer, works with any TCP protocol, nothing WebSocket-specific in it.
You should ideally design your messages to fit within a single Ethernet packet, so 2 bytes is more than enough for the size. Though I have sadly seen an increasing amount of developers send arbitrarily large network messages and not care about proper design.