They are green amoebas with darker green spots on their bodies. They have a mouth, tongue, and one eye each. They all appear to be the same size.
Role in episode
When SpongeBob sees a sign near the Chum Bucket that reads "Free amoeba puppy with order of chum," he greets Plankton so he can see what is going on. Plankton tells him that he wants to sell Spot's puppies. SpongeBob offers Plankton to let him help have the puppies adopted.
At Barg'N-Mart, Plankton drives off two Sweetie Scouts and their bench with their biscuits so he and SpongeBob can make room for their business. SpongeBob suggests that Plankton should start acting more politely, but the latter does not seem to want to do so, evidenced by him making two of the Barg'N-Mart customers run away as fast as they can. Then, SpongeBob takes some of the amoebas, only to make a larger puppy.
Incidental 42 notices it and wants the puppy, so SpongeBob gives him one. A crowd of people are waiting to buy one, including Mrs. Puff, who remarks how cute those amoebas are. SpongeBob makes, with the help of his body, more of them.