As the colophon in the Autobiography says, the body type is 10 point set on 
 a 14-point slug (in the language of the old Linotype days)--thus, "10/14". 
There  are 72 points to an inch; 12 points to a pica. The typeface in the 
Autobiography  is Adobe's version of Garamond, an old-stype serif typeface 
first designed by  Claude Garamond in the early 1500s. He was called a 
"punchcutter," and his  typeface, in Roman form, is characterized by the small 
"eye" of the lower case  E's and A's. It has a nice, fluid appearance, as is 
clear in the  Autobiography.
 
The material set in "smaller" type is probably no less than 9 point. 
 
For a good contrast, use your computer's stock of typefaces to set the same 
 three or four lines of Roman (not italic or bold) in Garamond, Century  
Schoolbook or Bookman Old Style, and Times Roman. Caslon, although not on a  
lot of computers' stock, is another fine "face." Another family, "Bodoni", if 
 you have it, is best as a headline type and not for body. In your  
comparison, also note the length of the lines--some faces take more space,  adding 
pages (and thus cost) to a book.
 
And as an aside, when Sam Clemens was a boy, there was a need for numerous  
typesetters to get out a daily newspaper quickly. It was a job much like 
working  at McDonalds was a few years ago--a job boys could do after school 
or, as in  Sam's case, instead of school. It is not unusual for the 
biographies of men  who were young in the 1800s to include mention of their having 
been  typesetters.
 
Bob Stewart