Do you have needles appropriate to working with worsted weight yarn already? That should give you a pretty good idea what size needles you want for working in the round, your gauge should be similar in the round or flat. I only have longer interchangeable or fixed circs because they're more versatile. 40" (or thereabout) circs are great for magic loop and doing 2 @ a time socks. You could use 2 circs if that's better for you. Check out the Advanced Techniques videos in the free videos available at the top of the screen.
This is the last time I'm frogging this project for the last time.