For worsted weight yarn:

Cast on 85 using a provisional cast on.

Knit 10 ridges in MC, then 2 in CC, 14 MC, 2 CC, 14MC, 2 CC, 14 MC, 2 CC, 14MC, 2 CC, 10 MC.
This gives you 86 ridges, so when you pick up for the edge stitches, just miss one and pick up 85 st on each side.

This gives you 85 per side instead of 61. You work the repeat of the edging the same, you just have 2 extra repeats of the design per side.

Obviously, you don’t cast-off when you’re finished with the garter-stitch square. Just keep going after your last row and commence picking up edge stitches. Now you will be glad about that provisional cast-on because you can just slide the loops onto the other end of your circ and knit them as you come to them. Don’t forget that you need to end up with 85 st per side, with a marker at each corner.

For the edging, make the black stripes as wide as the white stripes to make the whole thing bigger (switch colors on round 11, then every 8 rounds after that).

Use Jeny’s Surprisingly Stretch Cast-off around the edge:
http://www.knitty.com/ISSUEfall09/FEATjssbo.php