With you images this is quite easy (even if you have several objects in one image correctly aligned).
The following macro converts the image to a binary, sets a rectangular ROI (with 1 px height) at 20% of the image height and then measures the bounding rectangle of the selection only with a particle analysis.
run("Set Measurements...", "min bounding redirect=None decimal=8");
run("Convert to Mask");
makeRectangle(0, mHeight, mWidth, 1);
run("Analyze Particles...", " show=Outlines display");
This macro itself can be applied to a folder of images with the default Batch Gui of ImageJ, see:
However there is so much more you can measure to compare the different artifacts.
With ImageJ plugins (see: MorphoLibJ - ImageJ) or in combination with R packages (see Morphometrics with R | Julien Claude | Springer)