In order to change the text size of answers you need to do this:
Look for file qa-theme/your-current-theme/qa-style.css
Search for .qa-a-item-content
If present then add this line between the curvy brackets: font-size: 25px;
If not present then add this to the file:
Of course, replace the number 25 for whatever you consider big enough. In order to change the font for questions and comments too you will have to perform the same steps with .qa-q-item-content and .qa-c-item-content, respectively.
Also note this will taint the theme and you will have to remember this change whenever you update the theme in the future.