IMAGE AND SPRITE SETTINGS

Hi to all, I'm starting to design an app, and since I am a rookie of unity, I have some important questions. 1 - The devices are many, but I'm trying to understand the mechanics of the images to be imported in Unity, that if I predict that my app can preferentially run on IPAD (for example), what kind of setting I have to give that amount to the images as sprites to be used in the UI as UIImage? 2nd - If I want to consider a maximum resolution of 2048x1536 which covers most of the devices Ipad I import my images to 2048? I also noticed that if on photoshop my image is 8-bit when imported from unity becomes automatically 24bit and huge as weight in Mb. Why? 3 - If I want my app to be enjoyable in any media Ipad the canvas should be set to "Screen space overlay" and the canvas scaler "scales with screen size" ... but at what minimum resolution? 2048x1536 or 1024x768 that regarding Ipad2 ?. 4th - What is the importance in terms of the technical setting "Pixels per unit" in the settings of the sprites? and consequently the canvas scaler should be reported the same parameter or are unrelated concepts? 5th - If within my app I predict that most of the content could be achieved only in 2D but I think there is such a chance to pull the dice that have a mechanical 3D, the entire project must be 3D? After this string of concepts that I have to make clear in my head I hope there is some good soul who has the patience to answer at least some of my questions. Thank you and good job.