In the API documentation at https://developers.digitalocean.com/documentation/v2/#create-a-new-droplet it says:
size String The unique slug identifier for the size that you wish to select for this Droplet.
You can list out the sizes through the API, and this will help you to find the slugs. You can draw correlation to the pricing page by the resource/price. The documentation for that can be found here:
Initial Server Setup