Never considered the size of the bubbles before, I just always turn it up and let the air flow. A big misconception is that the bubbles inject oxygen into the water which is not the case. The only place that gases exchange in water is at the surface of the water. The sole purpose of the bubbles is to break surface tension creating water movement at the surface. The more the water moves at the surface the faster gases exchange at the surface allowing faster and easier gas exchanges, like co2 out and oxygen in. Your fish need oxygen in the water to breath and they exhale co2 just like we do. Stagnate water has much slower gas exchanges at the surface.