Pretty sure nobody did a study with a specific number of LoC for best understanding, because it probably depends on a lot of things. But quick understanding is a function of consistent style and a small number of things to keep in mind required to understand each part.

Things are easier to understand when the deciphering part is done quickly and also when you only have to keep as few things in mind as possible.

An example of a Python statement that is short, but difficult to understand:

    '\n'.join([map(some_func, a) + 'somth' for i, j in c.items() for a, b in i.items()])