I did exactly this - knit a top down turtleneck raglan sweater with a big cable down the front. Here's a picture of it in progress:
I used Merino Chunky -- size 10.5 needles. I did end up starting it, getting a couple repeats down the cable and realizing that the back was growing faster than the front. Then I measured, ripped out and cast on again adding extra stitches in the front to compensate for the pull of the cable. I wrote it down - it's somewhere at home but if you'd like me to search it out for you I'd be happy to. I meant to put the pattern on my blog but never got around to it.