in unreal, hover your mouse over the imported textures, that shows you the filesize after compression. now try it out with a 24bit texture and a 32bit texture.
You don't need to worry about it in photoshop at all. Photoshop will give you a preview of the map in SRGB/gamma space, but what the map looks like in PS doesn't matter. What matters is how it looks in your game engine/renderer, and thats where you will check when authoring content. Only other thing to really keep in mind…
What it looks like in the engine is the most important bit. If you're previewing in say, Marmoset Toolbag, but the final engine is Unreal 4, you want to make sure that for all of your textures, you're using the same color space. In TB2, you can click on the per-texture options button and enable or disable sRGB. sRGB on…