Here are the specifications for label font sizes set by FDA:
1.) “Nutrition Facts” lettering must be bigger than any other font on the nutrition label (except for the numerical value for calories) and must span the length of the whole nutrition facts panel
2.) The smallest font sizes that can be used on the nutrition facts label are 6 point (for the lettering below vitamins, minerals, and footnotes) and 8 point (for the rest of the label).
3.) “Calories” lettering must have a font size of at least 16 point (or 10 point for small packages).
As far as label size, there is no minimum size set by FDA, however there are a few clauses for label location or type based on total package size.
1.) For packages with a total surface area of 40 square inches or less, the nutrition label can go on any part of the package.
2.) For packages with more than 40 square inches of total surface area, the nutrition label must appear on the information panel, PDP, or somewhere within plain sight of the consumer.
3.) If the package has less than 3 inches of vertical space for nutrition labeling, you must use the tabular nutrition label format or the side-by-side format (with the vitamin and mineral panel to the immediate right of the nutrition facts).