I have a coworkers who is overweight and I'm going to tell them to go on a diet.
What would make you want to tell them to go on a diet?
I find that it's very unconfortable for me to work with her and I don't want her to harm herself.
Her being overweight should not make you uncomfortable and you cannot tell her to go on a diet. It will make the work place uncomfortable and could lead to you being reprimanded and fired.
I don't think you understand what I'm dealing with.
It's not your place to add commentary and give unsolicited advice to your coworker about her being overweight as it's rude and out of line. You need to rethink what you plan to do as the consequences will most likely be negative for you, not to mention you will embarrass your coworker and hurt their feelings.