How much should a 58 inch girl weigh?

A BMI (body mass index) of between 18.5 and 24.9 is considered normal in the US and many other countries all though some countries might not follow the same guidelines. BMI=mass(kg)/height^2(m). A 58 inch girl would be considered normal weight if she weighs between 40kg(88lb) and 54kg(119lb).

This is a very vague answer as the BMI does not consider percentage of body fat or muscles so a weightlifter might be considered obese if following the BMI, but he might be very fit and not fat at all.

The BMI is just a guideline, the most important thing is ones health and happiness :)