How can I calculate the growth of a small creature using a DSLR?

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I have a science project idea to complete in which I intend to grow brine shrimp and collect their growth data.

I am planning to photograph these creatures daily from a set height using a clamp. I will take images from above their open topped tank to prevent distortion.

However, I am unsure as to how exactly I am to calculate the growth from these images taken. Perhaps I could use trigonometry? Although, I am unsure how to use it in this way.

Any help or suggestions would be greatly appreciated.