Note that in my experience, socket systems treat the buffer sizes as more like... rough guidelines. Especially Windows and Linux. Which doesn't mean they're not useful to expose somehow, but you can't assume that just because you set the buffer to size X means that operations will start blocking after X bytes. When trio's tests need a clogged socket, we just keep sending in a loop until we observe a BlockingIOError, and then do the test.

Pipe buffers are much more reasonably behaved IME.