What weight & BMI should I be at 5'9 & 28 year old female?

BMI. Bmi is calculated weight to height. Ideal and high risk are standardized for adult ideal is18.5 to 25, 25 to 30 is overweight and 30 plus is considered obese. These are not generally age dependent. Another way to determine ideal weight for a female is 100 pounds for 5 foot and 5 pounds per inch for every inch over 5 foot. There are allowances for large and small bone. Hope this helped.
Check this out. Use a BMI calculator to know. SEE: https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm. BMI Categories:
Underweight = <18.5 * Normal weight = 18.5–24.9 * Overweight = 25–29.9 * Obesity = BMI of 30 or greater. By checking your BMI you can get an idea of whether you are in a healthy range.