A big wave is any wave that scares you.
Many years ago (when I was diving and not kiting) I took my 'rubber duck' (4 metre inflatable with 30hp motor) out for a spin at 'Blackfellows' surf break, near Elliston during a huge swell.
We headed into one wave, it was like driving up a big hill, seemed to be going up and up for ages on this huge green monster. As we neared the top, the top few metres started to break, we had just enough time to turn the boat around and race back down the face! Exciting stuff but we didn't stick around to try our luck a second time

I don't have any photos and wouldn't be game to try to guess the size, but this was on a day when the BOM were calling a 6 metre swell. Anyone who knows the region will have a good idea of what the break would look like in that sort of sea.