I am creating a magazine. What is the best typeface and font size to use for content?
Your question is a bit broad so here are some broad guidelines:
First you need to consider your target audience when defining the font size. Typically, a magazine could go from 8-11ish point size for its main text but this really depends on the x-height of a font. The x-height is the height of the lowercase in relation to the height of the uppercase characters. Different fonts at the same point size will look much smaller/bigger depending on this factor. For example, Helvetica looks huge at 9pt when compared to a lot of other fonts. Personally, I also like to ensure that my main text font features true italics (easy to tell by looking at the a, which usually varies its shape in a true italic while, in a "fake" italic, or what would be call an oblique, the a merely looks bent. I find true italics are just easier on the eyes.
Second, you want a font that will provide a multitude of weights and glyphs. You don't want to find out at the last minute that you don't have access to italics, bold, small caps and such, or some glyphs you will typically need (e.g. you are laying out a magazine in French and don't have the proper accents, or you are laying out a magazine related to math and the mathematical subset of glyphs is lacking).
Third, you try to make a relevant choice with regards to the topic at hand but make sure not to pick something that is decorative in nature. In doubt, stick with what is most legible.
Emile had great advice. I'd add that serif fonts are often thought to be easier to read, so you might want to consider a serif font for your text, which you want to be inviting to the reader. Once you choose your font, letter and word spacing help with the visual appeal of your text, as will subheads to break up the copy, and lure the reader on. Sans serif fonts work well for headlines and subheads. Consider going to look at lots of magazines (library, Barnes & Noble), and see what you like, what would work with your magazine's subject matter. "Scientific American" for instance, uses serif for text and heads; The New Yorker, also sets type in a serif font, but uses its own sans serif font for the heads. Best wishes!