i dont think there is any real "proper" length but you want to try and get your shock to be in about the middle of its travel at static height.
that way you have equal compression and extension.
essentially so long as the shock never tops out, or bottoms out, its alright.
so measure how long the shock is at static height (which i think you already have), and use this as the basis to work out what shock suits your height. use this figure as the mid point of the shock compression. it will take a bit of thinking, but wont be too hard. you will need the compressed and extended lengths of various shock lengths.