"pala leipää" translates literally word to word to a piece of bread, so it means anything that's only a part of the whole bread (bread loaf). "bread loaf" can be translated to "limppu", but I think we use that only of certain types of bread. if you want to say that you'd like a small bread loaf for example you'd usually say "pieni leipä, kiitos".
Also: note the difference between "leipä" and "leipää". the latter is the same word in the partitive case, which already implies an uncertain amount of bread.