Node and socket.io don't have any built-in limits. What you do have to worry about is the relationship between the size of the message, number of messages being send per second, number of connected clients, and bandwidth available to your server – in other words, there's no easy answer.
Let's consider a 10 kB message. When there are 10 clients connected, that amounts to 100 kB of data that your server has to push out, which is entirely reasonable. Add in more clients, and things quickly become more demanding: 10 kB * 5,000 clients = 50 MB.
Of course, you'll also have to consider the amount of protocol overhead: per packet, TCP adds ~20 bytes, IP adds 20 bytes, and Ethernet adds 14 bytes, totaling 54 bytes. Assuming a MTU of 1500 bytes, you're looking at 8 packets per client (disregarding retransmits). This means you'll send 8*54=432 bytes of overhead + 10 kB payload = 10,672 bytes per client over the wire.
10.4 kB * 5000 clients = 50.8 MB.
On a 100 Mbps link, you're looking at a theoretical minimum of 4.3 seconds to deliver a 10 kB message to 5,000 clients if you're able to saturate the link. Of course, in the real world of dropped packets and corrupted data requiring retransmits, it will take longer.
Even with a very conservative estimate of 8 seconds to send 10 kB to 5,000 clients, that's probably fine in chat room where a message comes in every 10-20 seconds.
So really, it comes down to a few questions, in order of importance:
- How much bandwidth will your server(s) have available?
- How many users will be simultaneously connected?
- How many messages will be sent per minute?
With those questions answered, you can determine the maximum size of a message that your infrastructure will support.
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