I googled “Optimal font-size”, read first 16 pixels then a dao of web design and came to the conclusion that the optimal font-size is not to specify one.
As for optimal line-height - around 1.5em is common, but The Golden Ratio seems a little neater. So that’s a good standard.
As for line length - golden ratio typography
claims that it should be proportional to line-height, which makes sense, but the equation they suggest
just doesn’t make any sense.
A Smashing Magazine article
suggests that around 65 characters per line is ideal, but that most people go for more like 75.
65em / 1.618 (golden) = 40.173 so I’m going to suggest that 41 times line height.