Garlic Cloves Size Differentiation
A clove is not a clove. There are 2 main types of garlic: 


Hard Neck: hard neck garlic varieties have larger bulbs (or head as you call it), and fewer, larger cloves
Soft Neck: soft neck garlic are generally smaller than hard necks, and have more but smaller cloves


Hard neck varieties are on the whole stronger flavored than soft neck varieties, although a lot of it depends on the richness of the soil and how well it was farmed. All garlic varieties have larger cloves on the outside and smaller ones on the inside. 

As for how many to use, if a recipe calls for 8 cloves I generally treat one large hardneck clove as 2-3 cloves, and softneck cloves on a 1:1 ratio. Some of the varieties I grow are very strong, so if I'm using a red duke I may just use 1 clove for a recipe that calls for 8. Generally the rule I gave will work fine for store bought garlic.