How to determine the true size of a font?

Adobe Acrobat professional: Tools -> Advanced Editing -> Touchup text tool. Select text, right click -> Properties. Text tab, Font Size box.

PDFedit will be able tell you what the size of particular bit of text in a PDF document is.

There are several definitions of the word “point,” and LaTeX itself has two different units, the pt and the bp (for “big point”).

Since you say that’s a minimum size, the easiest solution is probably to load \documentclass[11pt].

With fontspec and a modern TeX engine, you have the option of setting \defaultfontfeatures{ Scale = MatchUppercase } and then loading your main font with \setmainfont[Scale = 1.042]. (Or 4.2% higher than whatever it’s at now.) This should scale your main font to the size the publisher requested, and scale all the other fonts in the document to the same height.

If you’re using 8-bit fonts, many legacy font packages support a scale= option.