"She has one little bread."
I'm not sure if your use of 'seul' in the sentence is grammatically correct but in any case, the word 'seul' means 'alone' or 'only', and in the sentence it is simply 'one'. Indeed "she has only one little bread" and "she has one little bread" convey the same facts about how much bread she has but they are different sentences with subtly different meanings and different French translations.
This is a good overview: http://french.about.com/od/grammar/a/adjectives_4.htm
This would be like saying she has a small amount of bread. "Un petit pain" is the term for a dinner roll. Notice the adjective "petit" describes the bread itself, the bread is small, not the amount of bread she has. If you say Elle "a un peu", you said, she has a little quantity of bread. This may seem unimportant, until you need to say "Elle a 20 petits pains" that's a lot of small breads!
I do not think so, a baguette is very long. Though a baguette is also referred to as Pain Long I think they make petit baguette (google that and you will find images) but I think they are called petit baguette not petit pain. I think there is a difference. As a baguette is stick shaped. I am going to Paris in a few weeks I will ask a bakery.