How do I choose the correct size silicone sheet?

Get a size or shape large enough to completely cover your scar and leave 1/4" around the area.

The best answer is to make sure you get a size or shape large enough to completely cover your scar and leave 1/4" around the area. You can refer to the sizing chart below to match some of the most common procedures for each silicone sheeting size. 

Scar Fx Shapes and Surgeries
1.5" x 3" (3.75cm x 7.5cm) Mohs, Trocar, Small Incisions/Excisions

1.5" x 5" (3.75cm x 12.5cm)

Chest, Orthopedics
1.5" x 9" (3.75cm x 22.5cm) C-section, Orthopedics
1" x 12" (2.5cm x 30cm) Arm Lift, Tummy Tuck
1" x 22" (2.5cm x 55cm) Tummy Tuck, Thigh Lift, Bariatric
3" x 5" (7.5cm x 12.5cm) Burns, Traumatic Scars, Face Lift
4" x 8" (10cm x 20cm) Burns, Traumatic Scars
3" Breast Circle  Breast Augmentation (Areola Entry)
Breast Piece Breast Reduction (Under Breast Entry)
Breast Lollipop Breast Reduction or Breast Augmentation
Breast Anchor Mastopexy, Breast Augmentation, Breast Lift
1.5" x 3" Breast Sheet Transaxillary, Breast Fold