I have a large map in my game and I have separated it in 14 parts, which I made them all 900x1145 big by mistake. That froze my phone and I have reduced it to 450x573. But the game's performance is still not ideal and I think additional reducing of the image sizes would screw the quality of the map for better displays...
Should I consider dividing my map image into another 14 parts (28 in total) ?
What is the minimum size of an image that will look nicely on every display without causing the game to lag?
Phones often need to process images as power of 2 sizes (2048, 1024, 512, 256, 128, 64, 32), so it will upsize to the next power of 2 filling in the rest of the image with blank space.
This means your 900x1145 will be treated as 2048x2048 which takes 16MB and your 450x573 will be treated as 1024x1024 which takes 4MB.
If you can, break the large map image up into pieces no bigger than 512x512 (which only takes 1MB each), although 1024x1024 may work just as well depending on layout dimensions. For example, if the layout height is 2048, then two 1024x1024 images would work just as well, if not better than 4 512x512 images, but if the layout height is 1536, then 3 512x512 images would work better then two 1024x1024 images.