Texture file sizes

Im optimizing my textures for the first level of my game. So im wondering for a game with only diffuse textures,
using light map. With a poly count of max 5k per frame. How big should my texture files be. Can someone give me a ball park figure thats safe for the average game? Currently Right now all my textures together are about 2.5mb based on the resource file value in UE. But i still need to add some assets. I suspect im way under the “limit” but
id like to know how far i can go as i will need to build some enviorements with alot more detail soon. The taget is for the game to work on iphone 5 or 5s and forward.