You are probably calling Sprite.GetSize() somewhere, but this function doesn't exist.
You probably found the function in the sfml 1.6 tutorial, but it seems to be removed in sfml2.
After a quick look in Sprite.hpp I think you will need GetTextureRect.
This function is a little bit different (it returns a rect instead of the size), but I think it will give you the same result.
Otherwise you can call Texture.GetWidth() and Texture.GetHeight() to find out what the size of the texture is.