Maybe it's a stupid comment, but when I increase the size of the picture, it gets blurry. What should I do?
It's pixel art, you can't really increase the size. Look for pixel perfect cameras in your game engine, and use the sprites that way.
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.