The easiest way I've found to sort out how wide a sleeve should be is to loop a tape measure loosely around my arm until it looks like the right size for the period and character. Do this with your arm held out to the side, or you won't be able to really see what's going on. A mirror is a help, as well.
Start with a line. As with the body of the chemise, this first line will be placed on the fold when cut.
On this first line, measure and mark the length of your sleeve. For a full sleeve chemise, this measurement should be no less than your shoulder to wrist distance, and no greater than your shoulder to fingertip distance.
Ok, that was less than precise – this is really more of an art than a science, no matter how hard the Victorians tried to quantify everything. Here’s a sneaky trick: if you want a chemise sleeve to look very full, you make it a little longer than you need it. That gives you that romantic I-just-have-so-much-sleeve-I-don’t0know-what-to-do-with-myself effect. But you don’t want to end up with so much sleeve that you can’t use your hands, either, so it’s important not to make the sleeve longer than the entirety of your arm. If you’re planning this for a specific era where chemise sleeves were poofed out of slits in the sleeves of the outer garment, you might want to make the chemise sleeves longer. Just be mindful of the danger of looking like a child raiding an adult’s wardrobe…
Also, you might have noticed that I’m drafting the sleeve slightly different to how I drafted the body. The truth is, there’s more than one way to do things… Some people like to start with a corner and draft out from there, some like to start with a line, heck, sometimes I start in the middle. If one or the other method seems more intuitive to you, just use that one.
Measure out 1/2 of your sleeve width from your original line. Complete the rectangle by drawing a second vertical line at this point.
Optional: if you took a corner off of the chemise body to reduce the armscye bulk, you should do the same thing to the sleeve. Ideally, these triangles should be identical.
As before, we want to add seam allowances, mark that this is a sleeve, note the edge that goes on the fold, remind ourselves to cut two, etc.
A simple chemise, sewn with drawstrings at the cuffs and neck.
If we sew it a little differently, this pattern can make us a square necked chemise.
And that is the doll-sized version of my favorite festive peasant chemise. It bears no resemblance to actual historical fact, but I can tie it up at the shoulder on a hot day, and a lot of people seem to mistake it for something they remember seeing somewhere, in some book, maybe, but it certainly looks familiar. It's a little Disney like that....