There's a canon tale/legend about a guy named Illyius who had a mouse patronus and, with it, saved his who village from a dark wizard. (According to HP wiki, it was told in "Wonderbook: Book of Spells" and briefly mentioned on Pottermore)

Anyways, what I meant was that yeah, the size of your patronus doesn't make it less powerful. Luna's patronus is a hare, McGonagall's (we can all agree that she's a very powerful witch) is a cat. Neither of these are big animals but they still defeated dementors.

As I understood, the only thing you need to make a powerful patronus is concentration and to very clearly see a happy memory, to be able to feel it. For example, even Harry, as we know to have a powerful patronus, sometimes had a hard time. During the Battle of Hogwarts, at one point there's a flock of dementors coming and with the weight of everything, he couldn't form his patronus. It was only a feeble thing that faded in an instant, Ron and Hermione's too. Then Seamus, Luna and Ernie came to their rescue.