@Mike: thanks for your reply.
When I wrote "correct size" I simply meant the scale, not the aspect ratios. To be more precise: the sprite I bought came in three PNG scales 300 px, 600 px and 1200 px high. In my game, I need the sprites to be 200 px and 400 px high. I also got the AI versions of the body parts and I wanted to "rebuild" the character in the sizes I want and then use Spriter to get the same animation. If I understand you correctly, that is possible to do provided that the image are the same aspect ratio and are rotated in the same direction.