For both height and head size you should use a device that cannot be stretched, such as a tape measure or a piece of string with half-centimetre measurements marked on.
To measure your full height
Take off your shoes, socks, and any headwear. Stand with your back straight, preferably against a wall, and with your feet together. Either have somebody else mark the highest point of your head on the wall with a pencil, or place a box on your head, hold it there, and mark the wall underneath it with a pencil. Then simply measure from the floor to the pencil mark with a measuring tape.
To measure your head size
Wrap the tape around the widest part of the head. This is usually just above the eyebrows around to the widest part of the back of the head. Measure this at least 3 times and take the largest number.