What is the image size per layout?
In the manual, I know that 100mb per layout is the average,
I know it's a very old story.
Is there anything appropriate?
I'm about 2-300mb.
It depends on what platform you are developing the game for. 300MB or even 500MB should be ok these days, but it may run slow or even crash on some old phones.
I'm developing IOS, Android App.
300 - 500 MB will be okay?
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I'd say 500mb is pretty safe in recent devices within 5 years, outside the absolute bare bones lowest end budget devices.