What size font?

I write in 12-point Times New Roman font. As a writer, this is the most acceptable font for submitting items for publication. Of course, if you are designing a menu, brochure, or website, there are other choices to make for readability, style, the theme of your business, your targeted audience, etc. Most importantly, no matter what font, it is essential to have no typos, grammatical errors, or formatting snafus that will fail to convince or attract your audience. 

Basically, you want to use a font that is legible for your audience on the device/media they're engaging with your content on. You need to select a font that is clear and reinforces your brand identity. So, size is just one of the factors to consider, especially since size is very dependent upon the font you choose. (12-pt Times looks like a different size that 12-pt Arial, for example.) Serifs or no? Will it be used for digital or print or swag or some mix of these things? What color scheme are you using? These all impact the readability of your font. Consider, for example, how you'd adjust your font if your audience is older folks or people with colorblindness or English-language learners, or if your content consumers only view your copy on mobile devices.

Industry standard for book manuscripts is Times New Roman, 12 pt. If you plan on submitting to an agent, editor, or publisher, it's best to stick to that standard. Though most will post specific guidelines on their submissions page. 

I always use the size and font required by most publishing companies for submissions, 12 point New Times Roman. 

12 point, New Times Roman

As a freelance writer who works with a number of clients, I always go with what the client wants. After all, they pay the bill!

For my personal website, while I can make changes to the font, I prefer to go with what the website designers have incorporated into my WordPress content manager site. It seems to come out the best -- and my website maintenance/design team also designed the website for mobile access.

I want the reader/viewer to have the best possible experience, and not get bogged down with a font that I think may look cool but doesn't meet their expectations.

Customers always rule!

Times new Roman 12. Double space. 1 in margins. 
almost always

Always sans serif because it's easier to read especially for anyone who has any type of vision challenge. With serif typefaces, I generally bounce from the page because it's 'TOO MUCH WORK" to read. You always have to think about the reader's experience.

As to size, generally equivalent to 12 points is the minimum...again to include anyone who has vision challenges.

You always have to think about the user experience when you write on the web.

Even more important than font and font size is to craft copy that is scannable. Note that people do not read a web page when they first land on it. They only scan. If what they read in the scan fits what they are searching for, they will go back and read. 

For that reason, the headlines (H2 and H3) and bolded first sentences must tell your story with just a scan. Constructing those components of your copy correctly is the reason you can entice a prospect to actually read the entire page...and go on to the next page.

It takes a lot of extra work to craft scannable copy, but it pays off BIG TIME.

Suzi

i think it depends on the feel you want it to have. you can be creative. but it's always best to choose something that is easy to read as opposed to wacky ones obviously. I actually used Arial 12pt in my last book and it was very nice

Times New Roman 12 pt. 1.15 spacing