interesting idea, execution could be better.
actualy the lower arm is about 1.2 times the length of the upper arm.
its a nice piece however there isnt that much shading, but i guess the colour constraints limit how well you can shade a piece. one technique i often use if to nhave the image in one colour, or maybe with a hue shift so your effectivly making it in B+W but its more interesting. when you utalise a hue shift in that you can have several colours, since pure green is lighter than pure blue for example, or rather apears lighter. worth baring in mind for future pieces, when i have a limited pallete i try to order the colours from lightest to darkest and pixel from there, as can be seen in my first entry for this comp.
but ads i said the dithering is too provident and as such doesnt look that great, one thing worth baring in mind (if you ignoring what i said above) is to be selective about placing colours to area, the smaller the area it covers the less colours it useds, for example the skin have 3 tones while the white suit has two despite being a larger part of the image.
its a nice piece tho, and i wish you good luck in the comp =)