Another very important factor in readability is the text width. The focus should be placed not just one line in particular but it should also be placed on all of the text lines. All of the lines on the page should be large enough to provide effective content, while also considering the fact that reader’s eyes only want to move so far.
Based on several different usability studies, 55 to 100 characters is ideal for readability. To give you an idea of how many characters that really is, simply look at the content you are reading right now with the font set at the average web size. My website tends to push the maximum of this ideal limit because my entire website is primarily based on text content.
As mentioned previously, line width should be connected to the line height. So, if you have a sidebar with only 20-40 characters, you should increase the leading between your lines of text. Alternatively, if you have a very long width of text, I would first tell you to stop and reevaluate. Text that spans across the whole page has very few solid arguments of why it needs to be that way, and the vast majority of the time there is absolutely no reason. However, if you must do it this way, you can also increase the line height, but not by too much.