This is exactly what I was going to say. The flat's size really has much less of an impact of its effectivness than its location. The sharper the breakline off the flat, the better. Bass, especially the bigger ones, feed mainly in areas where they have quick access to deep water.
So let's say there is a flat that is 8'-10' deep and is 100 yards by 75 yards and the breakline drops slowly into 13' of water. Then you have a flat that is 8'-10' deep that is 10 yards by 20 yards but the breakline drops from 10' to 18' sharply. The qaulity fish will DEFINATELY be on this flat. It may be small but big fish need security and that gives it to them.
When fishing the flats concentrate most of your efforts on the breakline. Although there may be alot of activity visible on the surface over the top of the flat, most of those active fish will be smaller. The large fish will sit on the breakline, dart onto the flat, pick off a meal, and rush back to the breakline and will most likely suspend over it.
Topwaters, suspending jerkbaits, and jigs are awesome flat baits.
Also, most flats usually have some sort of vegetation on it. Where the vegetation forms its own breakline is another fantastic place to find quality fish. They sit on the edge, hidden in the vegatation and they pick off prey swimming by but they feel protected in the weeds.
Secondary points are smaller or less defined points located just after or near a "Main" point.