Select text by x-height allows you to select text by a font size expressed in x-height. The x-height refers to the distance between the baseline and the mean line in a typeface; this is typically the height of the letters x, u, v, w, and z. You can set a threshold for the text x-height expressed in points.
Note that this Action only can work if at least one of the characters to be measured (x, u, v, w or z) is available in the PDF.