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?

>>8
And netcat is just a non-generic, shittier version of socat.

>>6,1
If you don't want to block, why are you using blocking file descriptors?