Use a Brannock Device, if you have one available, as these provide the most accurate measurements. These are most commonly found in shoe stores, so if you're at home, you can also have someone trace an outline of your foot, and then measure the width of the drawing instead. You should be barefoot when your feet are measured, in order to get the most accurate results.
Stand with your feet parallel to each other. Place all of your weight on one foot, with your heel placed against the back of the Brannock Device. Measure the width of the widest part of your foot. If you don't have a Brannock Device, simply trace your foot, and then measure the widest part of the drawing. Measure your other foot the same way, as there can be a significant difference between the two.
Measure your foot without putting pressure on it. Compare this measurement with the first measurement. If there is a difference, you might have metatarsal compression. In other words, your foot is a different width when standing than en pointe. Consult with a ballet shoe fitter or dance teacher to find the best solution to making your shoes work if you have a metatarsal compression. Everyone has different feet, so there is no one solution that will fit everyone.