Check text x-height checks the font size by measuring the height of the lowercase x (= the 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 the check only can work if at least one of the characters to be measured (x, u, v, w or z) is available in the PDF.