If I want it to be compatible for both mobile phone, iPad and desktop? And for my assets, I made it rather big, eg. 500x500px for a small player on screen, and scale it to 30% in the layout. Would I have problem with game file size when I export my game in the end?
"Target aspect ratios, not resolutions."
Read this tutorial. it is excellent. I use 16:9 in all my projects.