Like Shanny said, it really depends on your font.
Size isn't the only factor either, leading is very important.
It's best to publish a test page and try and view it on a number of devices to see how well it reads.
For fonts that I use myself, I'm not really sure what size they'd be in the web-published file. I always put in type in my original large files and then size the whole document down. But as a basis, 8.5 x 11 in at 300 dpi I use 24 pt Humana Sans all in small caps.