what size should I use (resolution) to make a game for ios, android and windows phone? sorry for my english..
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You need to look at the common resolutions of each device, there are more than one size for each. Your best bet is to test different sizes and scaling settings in your layers. This way you can make the game one size and have it scale to each of the other sizes as needed.
Look at the tutorial for supporting multiple screen sizes as well. You don't have to build your game to a size for a particular device as long as it scales to that size without issue.