For those of you still watching this topic. The best tool to draw large circles with is a giant circle beam compass by Dascopro. Its not found in the craft stores so head to the hardware store.
Heres a product search on google for picture and range of prices. Lowest is $4.71 on Amazon:
Its great to use when making your cake bases, and hidden cake rounds. The largest round it can make is 10ft in diameter but it doesnt stop there....you can replace the rod with a longer rod (pvc, etc).