The length of reed inside your mouth, clear of your lips, must be free to vibrate at the pitch you want to produce. On most double reed instruments with which I'm vaguely familiar (not including folk oboe) the reed is sized so that as a first guess the length that extends inside you - and is free to wiggle - is about 1 to 1.5 times the width of the reed at the tip. That's a very rough approximation, but may give you a starting point to get a steady sound.

As you gain experience, you'll be able to control the volume sometwhat; and with lots of practice you'll likely get so you hardly hear it enough to bother you.(?????)

John