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It's really hard to give you a step-by-step hand tutorial on every single hand position out there. On deviantArt, if you search for "hand tutorial" there are a bunch out there with really simple step-by-step anime-styled hands, but only for a few select positions.

What you should do is just use your own hands or photos for references. Don't try to copy them line for line, think about how the hands are constructed and try to draw through them. Break them down to simple shapes, like tear-drop shapes and circles.

As for making them the right size, I always draw a very light, simple soft shape to represent the hands. If it turned out too big in comparison to the rest of the body, I erase and re-draw them (same goes with making them too small). It's just a matter of trial and error.

Also, practice them constantly!