Short answer: never. You can't be moving at the speed of light.
Long answer: if you ignored relativity
circumfrence / day = c
circumfrence = radius * 2 * pi
radius = c * 2 * pi * 1 day
Using google as a calculator:
http://www.google.ca/search?q=c*pi*2*60*60*24+seconds
I get:
1.62747495 × 10^14 meters
or, roughly 25 516 610.8 times larger (in every dimension) than it is right now.