I'm definitely no expert, but cuffs are the end of a sleeve that is typically trimmed with a band of fabric, sometimes adorned with a slit and buttons. In this case, it looks like the cuff isn't tight around her upper arm, which means that the sleeve doesn't taper in at her elbow. In fact, if you want the cuff to have a looser, oversized appearance when folded back than your sleeve would need to expand in width towards the cuff. You'd need to make sure that the widest part of your cuff is wider than your own upper arm, but how wide is up to you and how comic and exaggerated you want the appearance. Remember that the bigger the cuff and the wider the sleeve, the less structured it will be, and it might even be floppy. So if you want it to hold its shape you might want to consider using an interfacing. Also, a cuff that is folded over twice will be more stiff than just one fold. If that is your intended goal, you have to make the sleeve long enough that with 2 folds it still has the desired length.
I'd examine a men's dress shirt to see what finished cuffs look like if you're unfamiliar; they're a wide band of fabric that is folded over the unfinished edge of a sleeve and then stitched down. So then the edge of the sleeve would have 3 layers of fabric once the cuff was installed, and this keeps the dress shirts structured and from having a loose drape at the wrists.