Using a blood pressure cuff that's too large or too small can give you inaccurate blood pressure readings. Your doctor's office should have several sizes of cuffs to ensure an accurate blood pressure reading.
When you measure your blood pressure at home, it's important to use the proper size cuff. The inflatable part of the blood pressure cuff should cover about 40% of the distance around (circumference of) your upper arm. The cuff should cover 80% of the area from your elbow to your shoulder.
If you're concerned about the size of your cuff or your blood pressure readings, talk to your doctor. You may need to have your blood pressure taken several times during your visit using the same, properly sized cuff in the same arm. An average of these blood pressure readings should give your "true" blood pressure for that visit.