Tablets in  Size 1 pack are for very small dogs that are under 6kg in weight e.g. Chihuahua, Miniature Yorkshire Terriers, Miniature Jack Russells etc. 
Please weigh your dog to establish the correct dosage. 
Although a Westie is shown on the front of the Size 1 pack, most Westies in our experience weigh between 6kg - 8kg. Size 2 would be better for them. 

Johnsons Easy Dose Wormer Size 1:
A single-dose treatment to kill both roundworms and tapeworms.
For small breeds of dogs and puppies up to 6kg.
Contains 3 x 100mg tablets.
Each tablet treats 2kg of body weight.

Johnsons One Dose Easy Wormer Size 2
A single-dose treatment to kill both roundworms and tapeworms. For medium breeds of dogs and puppies from 6kg up to 20kg.
Examples of breeds that weigh 6 - 20kg: Springer Spaniel, Small Collies, Cocker Spaniel, West Highland Terrier, Whippet etc. 
Contains 2 x 500mg tablets
Each tablet treats 10kg of body weight. 

Johnsons One Dose Easy Wormer Size 3
A single dose treatment to kill both roundworms and tapeworms. For larger breeds of dogs and puppies from 6kg up to 40kg.
Examples of breeds that weigh 20kg - 40kg: Labrador, German Shepherd, Larger Collies etc.
If you have 2 smaller dogs to treat you can also use this pack e.g 2 x Springer Spaniels.
Contains 4 x 500mg tablets. 
Each tablet treats 10kg of body weight.

Johnsons One Dose Easy Wormer Size 4
Multi-pack size or for homes with more than 1 dog. A single dose treatment to kill both roundworms and tapeworms. For large dogs from 6kg-80kg.
This is the pack if you have several dogs in your household or if you are the owner of a giant breed. 
Examples of giant breeds: St.Bernard, Large Rottweiler, Pyrenean Mountain Dog, Great Dane
Contains 8 x 500mg tablets
Each tablet treats 10kg of body weight. 

Please do not treat nursing bitches for tapeworm if she is still lactating.