Decrease Sprite Atlas max size at runtime on android device

Hi, I want to decrease max size of Sprite Atlas at run time on android devices because some android devices with 512 RAM cannot run application smoothly or application may crash on those devices. So how can we change the max size at runtime according to device specs. I also tried setting Quality level but it does not affects Sprite atlas used in UI of Unity 4.7.1 it changes terrain textures and other object but UI sprites quality remains unchanged. Any help will be appreciated in solving that problem.