A magnanimous person has a generous spirit. Letting your little sister have the last of the cookies, even though you hadn't eaten since breakfast, would be considered a magnanimous act.
Magnanimous comes from Latin magnus "great" and animus "soul," so it literally describes someone who is big-hearted. A person can show that over-sized spirit by being noble or brave, or by easily forgiving others and not showing resentment. It implies superiority, and is something you should say of others rather than of yourself. Being magnanimous doesn't require doling out tons of cash — just being an understanding and tolerant soul will do the trick.