While holding a deep breath, measure around your chest with the measuring tape under your arm pits. The tape should be parallel with the floor and NOT pulled tightly.
Now add four inches to your chest size and this should give you a nice fitting with room for a couple of layers of clothing underneath. If you plan not to wear clothes underneath simply add just two inches to your original chest size you measured.
Most one piece leathers are sized by the chest so the above goes for both one piece leathers and jackets.