The studies I've seen about readability and legibility tend to focus on a specific set of metrics: size, not just the point size, but things like the size of the lower case letters as a proportion of the overall letter height, and line length. People simply can't read really small type set in really long lines.
Michael BierutTarget for example, is just a dot with a circle around it, that's all it is, so if you want a logo like Target, you don't need to hire a designer, you barely need to know how to operate a computer program, the logo may as well be anything.
Michael Bierut