It could mean "but when I/he/she was small". I'd assume in this case "era" would mean "I was", but I can't be sure.

Welcome, both questioner and answerer.

Herman is right, sara. If you are familiar with verb conjugations, you can see this by going to the Conjugation tab and typing in "ser." Era is first person or third person singular of the imperfect indicative tense, which typically refers to an indefinite/unspecified period of time in the past.

Just to get in the practice of avoiding word for word translations, should we mention the obvious, that, even though, it could mean when I/you/he/(it) was small literally, it can also has the connotation of when I/you/he/it was very young.

pequeño (to correct your spelling)

and just to quibble, it can't mean she or the adjective would have to be pequeña
Neither can it mean I//it if the author/speaker/object is female (sex) or feminine (gender).

Just to get in the practice of avoiding word for word translations, should we mention the obvious, that, even though, it could mean when I/you/he/(it) was small literally, it can also has the connotation of when I/you/he/it was very young.

pequeño (to correct your spelling)

and just to quibble, it can't mean she or the adjective would have to be pequeña

Neither can it mean I//it if the author/speaker/object is female (sex) or feminine (gender).

Great spot qfreed.

What does the word era mean in the phrase 'pero cuando era pequeno..' ?

Era means I/ he or she was so the sentance could be saying: 'but when I was
a little boy' or it could be saying 'but when he was a little boy' depends on the context of what was said before it, hope this makes sense.
Ken.