Diffusion experiment with dialysis membrane bag
You can double-check the area after you're done:


Poke a small hole in the membrane and drain it completely.
Stretch gently by pulling the clips (don't let them slip) until the area between them is rectangular.
Multiply that area (L x W) by two, because the membrane has two sides around the now-flattened middle.


There will always be some small error, because the wet membrane can be stretched somewhat, but that should be insignificant compared to other sources of inaccuracy, such as edge-effects where the clips narrow the membrane, impeding flow.