Actually the best answer for this question comes down to your personal preference. In other words, there is no such thing as the best drape length in the world. However, to make the drapes look nice while playing their due role, you have to follow some conventional rules, if possible, and combine some of your imagination.
As known to all, you can’t make the curtain shorter than the span of the window you want to cover (presume that you have no special requirements here). So, basically you have only two choices – either make it floor length or window length.
Window-length drape refers that the length of your drape can barely cover the window height. This is a nice option if you need to open and close the curtain at frequent intervals. Or, it is especially true when something, such as furniture and radiators, are right beneath the window frame and you don’t want to make any changes in the decorations or furniture placement.
Floor-length drape refers that your drape just touches the floor. A lot of people like this style as it tends to make the house look more formal than the window length. Besides, the drape that goes all the way to floor is more luxurious. So, this is a more popular choice if budget allows.
Nevertheless, it has to be mentioned that these two types are far from infallible law. They are just two common examples for your reference. Actually you can make it any length between windowsill and floor, if you like, or more extra inches (puddles) than the floor length to make it look more elegant.
Repeat it again: it is up to you to determine the best drape length for your living room window no matter what size your window is.