Store the scale value in your Enemy class. That way, each instance of your Enemy objects will have their own scale value.
If you do not have an Enemy class, you should create one. You can also store the enemyTexture, and have each Enemy have their own appearance.
Edit
You most likely are not using the proper scale value in your Draw method. Take an example Enemy class:
class Enemy
{
protected float Scale { get; set; }
protected Texture2D Texture { get; set; }
protected Vector2 Position { get; set; }
public Enemy()
{
Scale = 1f;
}
public void Hit()
{
Scale = 2f;
}
public void Draw(SpriteBatch spriteBatch)
{
spriteBatch.Draw(Texture, Position, null, Color.White, 0, Vector2.Zero, Scale, SpriteEffects.None, 1);
}
}
Usage examples:
Enemy A = new Enemy();
A.Hit();
A.Draw(spriteBatch);