Maybe it's a stupid comment, but when I increase the size of the picture, it gets blurry. What should I do?
The best method is to increase the size of the sprite in the game engine. In Unity this would be done either by just scaling the character or by selecting the character texture and lowering the "Pixels per unit". Another option is to resize the texture in Photoshop using the nearest neighbour algorithm, then it should not blur but might give poor results.