Because this is a sweater - even a loose-fitting one - gauge and size matter. This week, we're going to knit gauge swatches and make sure we're planning the right size.
Joji has provided instructions for both a seamed and seamless version. Before you knit your swatch, you need to decide which version you'll follow...because you need to swatch the way you'll actually knit the garment.
Undecided? Here are a few points to consider:
The seamed version is a front and a back knit flat, back and forth, knitting the stitches on the right side and purling the stitches on the wrong side. The front and back are seamed at the sides with mattress stitch. Because these two pieces are worked the same direction (from the bottom edge up to the shoulders) for the same length, it's the easiest kind of mattress stitch. This would be a great "first seaming project" (promise!)
I'm going to knit the seamed version (I know, you're shocked aren't you?!) for two reasons:
first, knitting and purling. The seamless version starts with 16 to 18 inches of stockinette in the round...just knitting (yep, poke me in the eye with a needle!); and
second, straight needles. I love them! For sure the stitches are going to be crammed on tight, but that doesn't bother me!
If you're planning to knit the seamed version, you should knit your swatch flat. Cast on 30-40 stitches and work 4-5 inches of stockinette (knitting the right side rows and purling the wrong side rows).
As I mentioned, the seamless version starts with a big tube...several hundred stitches of stockinette knit in the round. And that means just knitting. No purling. Many of us knit and purl at slightly different gauges. So if you're just knitting, your work might be smaller or bigger than if you're knitting and purling. Boxy is a loose-fitting rectangle, designed with about 25 to 30 inches of ease. It's big enough, then, and you don't need to make it bigger by knitting at the wrong gauge!
If you're going to make the seamless version, you should swatch "in the round". I promise it's easy (after all, it's just knitting and no purling!) and it's a helpful technique to know (think hats and socks and other seamless sweaters). If it's new to you, here's a great video (you only need to watch the first few minutes to learn the technique). You should cast on more stitches, say 40-50? and you'll still want to work 4-5 inches of stockinette; knitting all the rows.
In summary - for Boxy, I don't think there's a technical reason* to choose one version over the other. You should choose the version that makes you a happy knitter. We've got a lot of stitches and a lot of yarn ahead of us; let's make it fun!
And finally - I strongly recommend you wet block your swatch and weight it once it's dry to see how your finished piece might grow. If you want to learn more about gauge and swatching, check out this article; it's excellent!
Joji states (on page 2 of the pattern) "The biggest difference among the different sizes is the sleeve circumference [at the elbow]." Measure your elbow. That's right, put the measuring tape right around and see how many inches it is. Find the size whose Sleeve Circumference at elbow most closely matches (I'd go up if you're between sizes). Then make sure you're OK with the resulting Body Circumference. I think this probably only matters if your elbow measures between 10-1/2 and 11". The 10-1/2" sleeve is a 68" body circumference and the 11" sleeve is a 76" body circumference. Choose the better body circumference; you can always adjust the sleeve size if you need to.
*While I don't think there's a technical reason for Boxy, but there might be a technical reason for another project. We're knitting at a fairly loose gauge. The resulting fabric is going to be really drapey, which means the piece will have a tendency to lose its shape as you wear it. Side seams would help it keep its shape. If the sweater were designed with a lot less ease, meaning it really needed to "fit", I'd suggest the seamed version. But a big boxy rectangle? if it loses its shape, who cares?!