The smaller skull in the red chart is 15 sts across. The larger red one is 25 sts across and the chart 1 is 35 sts across (the one she really likes).
For an adult mitten, about 8" around the hand is allowed, so in worsted wool, (about 4 sts per inch), the chart will be almost half the hand, the top half, so it will work. For the other 2 charts, you would need to work with a very fine yarn, something that gets about 9" across. So a very thin sock yarn. But that doesn't make for a very warm mitten. So I'd go with the smallest chart. Maybe a scarf to match with the chart 1 design at the ends? Depending on how good a friend she is, you might want to stop at the mittens:-).
You want to do these on 2 needles because doing this sort of chart is called intarsia. If you do it in the round, you'll need to do it by what's called Fair Isle, and you're going to be doing an awful lot of carry of unused yarn on the palm side of the mitten. If you want to do that, that's fine, but I'd recommend the 2 needle methos, instead. All you do is have a main color and wherever the chart has the skull squares, knit with a second color. If one or the other colors is not used for more than 3 sts, you want to cross them over in the back so that you don't get big loops that will catch on the fingers. Also, when you are changing colors, stitch by stitch, bring the new color up under and over the old color so that they are crossed over each other. If you don't, the sections of color will separate.