Deeb,
According to the folks who make Pro-Mix, they recommend a pine or spruce bark size of approx. 1/2" as ideal. The Decorative Groundcover Bark I buy at Home Depot looks like this:
This is the size you want to find, if possible. Also, use ONLY bark and not something that is ground-up core wood, as this can leach and poison the tomato plant.
Raybo