It's not about Big Gardens, it's about a Big Birdwatch in gardens. Any garden, even our minuscule one which is all flagstones and pots. The point is to take an approximate census of birds to see how numbers are changing. They are an indicator of change as well as being wonderful beasties. Taking census of the bird population regularly can show what changes there are and contributed data to ecology. And contributing is fun.
Join in, you know you want to.