Creating Sprite Sheets for a 2D game Unity

I have a question regarding the method of creating a sprite sheet so that it will not consume to much memory and still keep graphics quality. I was researching about this topic and found that I should make my sprites be on the power of two, as far as I understood that is to create sprites like 512x512, 1024x1024, 2048x2048 etc. I have created two sprite sheets one is 1024x1024 and the other is 2048x2048, they are exported on 16bit but they still take to much memory. When I override the settings for android and set the max size to 1024 and I set it on compressed the sprite sheet takes 2.7mb, it takes the same memory size when the settings is on 16 bits and also it looses quality on both of those settings. When its on truecolor it takes 5.3 mb but it keeps the quality, but that size is to much. The other sprite when I override the settings for android set the max size to 2048 and set it on compressed it takes 10.7 mb, when its on 16 bits it takes the same amount, but when its on truecolor it takes 21.3mb, how do I need to export my sprites from photoshop in order to keep the quality and the size in unity will be small. Also is there a problem if I for example set the max size for an image less then its actual size? For example to set the max size 512 for a sprite sheet that is 1024x1024? I'm asking this because I did it and set it to truecolor it did not change quality, but I dont know if that will have an impact on the games performance or something else