On the 64x64 sprite I dig the lighting conveyed on the right leg in shadow (with rim-light around the right boot), plus the slight shadow wrapped around the left leg and the upturned front of the left boot. That all must have taken some time to look the way you wanted it to. There's a simple effective shadow to the right side of the body overall, from skirt to torso to collar to face and the highlight in the hair has a nice effective gradation to it. The palette overall is quite nice and reminiscent of a 32-bit game or a Psygnosis 16-bit game. I really like this 64x64 graphic.
To be honest I don't think the palette is used at its best with the other two graphics though. Regarding the 32x32 sprite -- the hair highlight makes the hair feel almost plastic in texture like an Amiga game's platform tiles, and the colours used to outline the ears and head clash with the collar-outlining colours (since the collar's outline is stippled with green and light green here, and the collar is rendered with vertical stripes; compared to the 64x64 collar which is mostly white on the left side of the body with a pale outline, causing little clash against the colours of the skin ramp). The legs seem like they were almost resized from a larger scale and left as is, with a backwards-L of an outline colour for the boot that seems hard to understand its placement. The left arm also has some banding on the left side and around the fist, and both arms seem to lack a strong outline colour - these graphics might look better on certain themes than others but I assume most people here are using the default Pixeljoint theme with the dark grey background, and it clashes with the colours chosen.
Regarding the 16x16 graphic - the pixie's face is normally covered by her hair. Here the colour chosen to outline the hair bangs, put next to the colour highlighting the hair, is arranged on the front of the head in a pair of 1x2 clusters, looking like eyes. There's also a 2x1 skin-highlight cluster that seems more like a set of teeth than conveying any form on the face itself. There's some good use of darker colours to outline the underside of the right arm and the side of the right leg, as well as separate the left arm from the torso. But the collar still clashes with the head.
It seems like some colours in each ramp will always clash with one another and there isn't a lot done to separate them in all three graphics; it's just less noticeable in the 64x64 graphic since its silhouette means the arms aren't forced to be positioned next to the torso.
Nice. I really like the 32x32 one! :D