Question about UNIX select()

I know that select() tells you if a file descriptor is ready for reading or writing without blocking, but how do you know how many bytes can be sent before it must be blocked? Is there some minimum in a header that I can rely on?

>>4
Well, thanks anyway, but it's not that important. It's just some homework I've already done, but it felt wrong to read and write only one byte at a time, so I wanted to optimize it. (The program is a simple server that writes whatever it receives to each connected client, and the clients send each typed character to the server and write any responses to the terminal.)