When I was little it seemed....?
Would this be imperfect because it's talking about my childhood? (Written like: Cuando yo era pequeña lo parecía...)
Here is a general rule of thumb for determining if you would use preterite or imperfect: If something answers the question "what happened" you will use preterite. On the other hand, if something refers to: what happened, the way things used to be, always were, what was going on, or just background information you will use imperfect.
Yes, it would be imperfect. There was no definite start or end time, it was just some time during your childhood.
Maybe this is a stylistic preference, but I would say:
Cuando yo era niña me parecía que...
I might be able to give you a better phrasing if you told me the complete sentence you are trying to form.