If one has a small forehead with a round face, cut the bangs in a way that when one parts them completely to the side, it appears as though the length goes from shorter to longer. Match this up with a medium hair length, wisped out away from your face or even straightened down, it will be gorgeous. What one might want to avoid is having longer bangs with this combination, because it tends to make the round face stand out a lot more.
If one has a small forehead with a heart shaped face, a pixie cut will look absolutely fabulous. Take for example Ashley Greene from the Twilight series as Alice Cullen. The hair she sported while playing this character looked amazing, and she has a small forehead as well.
With this combination of small forehead and heart shaped face, another cute hairdo would be having the hair straight down with your bangs a little longer, pulled and clipped back. Use an accessory, such as a fancy hair clip with a bump-it for extra volume, and one will look absolutely stunning.
With a small forehead, it is easy to cover up the entire area with bangs. Try to avoid that as it may be very unflattering.