Ailerons are plain flaps used at low speeds for lateral control. I would have to say that they are sized on roll rate requirements, and lateral stability at low speed.
They lose effectiveness at high AoA, and high dynamic pressures on aircraft with swept wings. Aileron reversal occours at high dynamic pressures, e.g. the 747, 727, 707 airerons are locked at high speed to prevent aileron reversal.
Most jets use use a combination of spoilers and ailerons at low speeds, and spoilers only at high speed. Spoilers are most effective with flaps down.
From memory the MU-2 has no ailerons....just spoliers
