How Big is the Universe?

I have never really ever thought about "How Big the Universe" was until my physics teacher asked that simple question, which I readily anwsered about 26 billion light years in diameter, but as soon as I have anwsered I knew I was wrong. What I was stating was that the observable universe is about 26 billions light years ( because we can look back about 13.4 billion light years), assuming that the earth in is the middle of the universe, but even if the earth is not the universe is so vast I really do not think that it matters. Anyways, we have a horizon (so to speak) of light that we can observe, but buy the time the light reaches the earth the universe has expaned from where the light has not reached us yet.