If it is fitted:
There are two measurements that are changed when going from a one size to the next in a race vest. Both the torso length and girth or dimensions around the chest and torso. The larger the vest, the longer the torso and the larger the girth. The best way to know for sure is to try the vest on with similar clothing that you will be wearing. Please note that more clothing will take space in the vest. Also a pack that is fully loaded will fit a little different that an empty pack.
Here’s my suggestions:
1. Put on the clothes that you will be primarily wearing under the race vest.
2. Load the pack with the contents or similar capacity that you will be carrying.
3. Put the vest on and see where the pockets fall across your chest and how much space you have in between the pockets. This is personal preference. Some runners like the pockets very close together. Others like them very far apart.
4. Attach the Cords on the Max O2 sternum and tighten enough to keep the pack from moving but not so much that it restricts your breathing. Most runners try to wear their packs too tight.
For one size fits most:
1. Put on the cloths that you will be primarily wearing under the race vest.
2. Load the pack with the contents or similar capacity that you will be carrying.
3. Loosen the side straps and Max O2 Sternum.
4. Put the pack on and position the front pockets where you would like them to be. Close together or far apart.
5. Attach the cords on the hook for the Max O2 sternum and adjust the Max O2.
6. Then tighten the side straps to snug the pack down for comfort and stability.
Practice accessing the pockets that are available while wearing the vest and imagine what it will be like after many hours of running in a fatigued state. This will be one thing that will help you determine if the vest fits properly.