On a standard 6" eInk screen (in this case, a Kindle), With font size about the same as a mass-market paperback, one gets about 10 words per line. With the font size a couple of steps larger, about the font size in a typical hardback, one gets about eight words per line. (Images below)
My advice would be to go for a waterproof 6" model, and see how you like it.
Most people go for a slightly larger font on their ereader because larger fonts are easier to read, and the extra pages don't cost anything. Paper books generally go for the smallest font they think their' customers will stand.
