Update: Chloe worked out some better numbers:

Cast on 85 using a provisional cast on.

Knit 10 ridges in MC, then 1 in CC, 15 MC, 1 CC, 15MC, 1 CC, 15 MC, 1 CC, 15MC, 1 CC, 10 MC.
This gives you 85 ridges, so when you pick up for the edge stitches, pick up 85 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.

(of course this uses more yarn than my numbers so you’ll definitely want to buy another ball of your main color!)

Here are my mods for worsted weight yarn (gauge 3.5 st/inch) I count garter ridges rather than rows:

CO 80 using provisional cast-on (the pattern doesn’t say to use a provisional cast-on, but it will turn out much nicer if you do).

Knit 8 ridges in MC, then 1 in CC, 15 MC, 1 CC, 15MC, 1 CC, 15 MC, 1 CC, 15MC, 1 CC, 8 MC (in other words, 15 ridges for the fat stripes and 8 ridges for the end stripes)

This gives you 81 ridges, so when you pick up for the edge stitches, pick up 85 on each side (pick up a few extra, evenly spaced, to make up the difference). The edge pattern is a 12-st repeat (plus one st at each corner) so this will work out perfectly and you won’t even have to think about the extra pattern repeats.

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.

To make the edging pattern look a bit nicer, I used both k2tog and SSK. Use k2tog for the two decreases before a set of YOs, and use SSK on the two decreases after the YOs. NOTE: I decided this isn’t as pretty as working the pattern as written. See my other Hap Blanket to see what it looks like as written.

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

Love this pattern! So fun and simple and fast. I used almost all of the four balls of off-white, and about 1.5 balls of green. Blanket ended up about 3’ square.