Recommended widths:

XS/Small Breeds- 5/8" 

Medium/Large Breeds - 3/4"

Large/XL Breeds 1"

^ Please note that these are suggestions, if your pet is well behaved on a leash you can consider using any width. If your pet is a known puller I recommended 3/4" or 1" with an Auto Locking Carabiner Upgrade (See Add Ons)