So I'm working on a game and in fullscreen it will look perfectly fine for 1920 x 1080 monitors but I'm wondering if I should spend time making it scale to other monitor sizes as well, would it be worth the time?
Preferably you should do this from the start of development. There are several other common monitor sizes in use, you can check the Steam hardware survey for one source of information.
There are several good tutorials on this site. You could start with these:
Absolutely! Making sure that your game looks great on any pc is fundamental to great game design. I suggest using the first link that @IndianaBones posted, however, both tutorials are great.
16:9 is the standard ratio. design your game at that ratio. 1080p is reasonable, it is the most used resolution and scalable with other common resolution of the same ratio