One of the basic ways to describe the types used for fifteenth-century printed books is the method refined by Konrad Haebler that involves, among other things, measuring the height of twenty lines of type. The height of a typeface affected its legibility, or how far a reader could be from a text and still be able to read it. The height of the type was also significant in relation to the other types used in a book, as a type taller than the one used for the main text often identified titles and other structural paratexts, while a shorter type was typically used for marginal commentary. So what at first glance might sound like a number only interesting to antiquarians turns out to have some interesting implications for the history of reading as a cultural practice.
With the availability of the Typenrepertorium der Wiegendrucke as an electronic resource linked to the Gesamtkatalog der Wiegendrucke, it's now possible to survey type heights systematically, so it should be possible to look at how type heights develop during the fifty years following the invention of printing. What may not be obvious is that we can do something similar for German manuscripts as well, as the Handschriftencensus records the height of the writing space and the number of lines for manuscripts where this can be determined - so we can divide the writing space height by the number of lines, multiply by twenty, and arrive at the "Haebler height" for each manuscript.
The boxplot below summarizes the means and 25th/75th-percentile limits for German vernacular mansucripts between 1351 and 1450 (with approximate dates coerced to a single year), and printed books separated by decade (with the 1450s and 1460s combined due to the small number of editions from the 1450s). While we're measuring what I think are comparable things, they're not precisely the same: the left column is looking at the line height per manuscript, while the other four columns look at the line height per occurrence of a give type - so a type used in five editions over fifteen years will be counted five times over two different decades. This is, I think, the best way to determine what a typical book might look like, with a frequently-used type counted more times than a type that was only used once. (To be completely consistent, we would also need to look only at books printed in Germany rather than all incunables.)
The next boxplot limits the y axis to make the picture clearer.