"I think that these shoes are small."
Translation:Yo creo que estos zapatos son pequeños.
Masc. singular: este
Masc. plular: estos
Fem. singular: esta
Fem. plural: estas
You can say "yo pienso", but I would say that in this context "creo" fits better. "Pensó" is past tense for the 3rd person singular (él pensó).
AND, "I believe these shoes are small" is a typical way of saying it in Florida (USA).