Try it and see how much space you save. If line length becomes unwieldy, consider increasing the right margin to reduce line length. Since browsers don't recognize the margin settings in word processing packages, I use a borderless table to control margins, and leave the right column blank. The size of the type you use will also be dictated by the background color of your document. White supports the smallest text, followed by gray.
Colored backgrounds require larger type for text to be readable.
And, even 12-point type can be illegible on a graphic or textured background. Sometimes what looks fine in your development package will be a lot larger, or smaller, once you get it online. All browsers display text differently. Unfortunately, we can't make the computer screen any bigger. As writers, what we can do, is make the best use of what we have.