FAA rules regarding number of seats in a row?

Does anyone know the FAA rules regarding the rationale of why in some flights there are 3 seats in a row and other times, 2 seats in a row, and in larger flights, 5 seats in a row? I travel often with my 2 kids and another adult will be with me, and it would be great to all sit together 4 in a row. The flights we take are smaller air crafts, so we have to split up, 2 and 2. In larger air crafts, there can be 4 or 5 seats together in a row. Is there a reason why, for example there couldn't be more than 3 seats in a row in a smaller air craft?