I just got really lucky with the circles. I got 2 Bigger squares, and then an invincibility. Becuase the square was so big, I just zipped around as fast as I could getting everything possible. The lucky part was getting another invincibility just as the first one ran out. By the time the 2nd invincibility ran out, the square literally covered the whole screen.
I don't know how making your square bigger is a good strategy. When you get invincibility it's easy to get everything on the screen anyways. I usually get pretty much every black square on the screen in the first 30 seconds, and that only gets about 2000 points.. what gives?