Sizes: 

Small: Length 90cm, Width 19cm, Waist 60-75cm
Medium: Length 100cm, Width 19cm, Waist 75-95cm
Large: Length 110cm, Width 19cm, Waist 85-105cm
Extra Large: Length 120cm, Width 19cm, Waist 95-115cm

Using a back brace posture corrector involves several essential steps.

1. Size Selection: Choose the correct size for a comfortable fit.
2. Clothing Choice: Wear lightweight and breathable clothing underneath.
3. Positioning: Align the curved part of the brace with your spine's natural curve. Place the wider section at your lower back and the narrower part at the upper back.
4. Waist Belt: If your brace has one, secure the waist belt snugly without over-tightening.
5. Strap Adjustment: If adjustable, set the straps with gentle tension. Focus on shoulder straps to enhance posture.
6. Comfort Check: Move gently to ensure comfort and proper fit.
7. Gradual Usage: Begin with short wear times, then gradually increase duration to prevent discomfort.
8. Posture Focus: During use, concentrate on maintaining correct posture.
9. Removal: After use, unfasten straps and remove the brace. Stretch and move to alleviate any stiffness.

Remember, wearing the brace is just part of an overall posture improvement strategy, which includes exercise, ergonomic adjustments, and posture awareness. If any discomfort occurs, consult a healthcare professional.