QualitySettings.masterTextureLimit

Description

A texture size limit applied to all textures.

Setting this to one uses the first mipmap of each texture (so all textures are half size), setting this to two uses the second mipmap of each texture (so all textures are quarter size), etc.. This can be used to decrease video memory requirements on low-end computers. The default value is zero.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void Example() {
        QualitySettings.masterTextureLimit = 1;
    }
}