Making realistic goals is an important part of creating a healthy body image. how does one determine if a goal is realistic?

A realist goal must be SMART:
SPECIFIC: instead of saying you want to lose weight, you must have a target weight (based on the ideal range of your BMI, i.e. between 18 and 21kg/m²);
MEASURABLE: You should be able to track your progress by your body composition, weight, body-fat distribution, so on.
ATTAINABLE: If you're 6ft tall, don't expect to weight the same as your 5ft friend.
RELEVANT: Without the proper mindset (i.e. fully understanding the reason why losing weight/ or building muscle mass is going to positively affect your life) you won't go very far. And even if you achieve your desired body, don't expect it to last. 
TIME-BOUND: So you expect to lose 50 pounds in one week? Not very realistic, is it?! The same goes to not having an estimated deadline at all. Set a date in a realistic future by which you should have achieved your goal. This is more of a motivational tool than anything else.

I hope this helps!