Uh-oh, wait, are we talking about initial visits (SEO) or overall long-term traffic including multiple pages, bookmarks and recommendations to other humans? A robot doesn't have eyeballs, though it can certainly read CSS. Unless you've expressly forbidden it to do so.
Postulate: Humans will spend more time on a site if its text content is comfortable to read.
The above seems intuitively obvious-- but things that seem intuitively obvious aren't always so. :( Has anyone got hard evidence?
I don't believe it would affect SEO at all. If you use header tags correctly, you may get a little boost but this is so old-school that it has very little - if any - affect on the rankings. And header tags (and even using <strong> ) are more likely to help over larger font size.