Oops, I forgot about sf::IntRect. Anyways, say you have a 100x100 square. You want to crop the image to a 10x10 sprite in the middle. You would do this:
sf::Image image;
if(!image.LoadFromFile("100x100.png")
{
return 0;
}
sf::Sprite sprite(image);
sprite.SetSubRect(sf::IntRect(45, 45, 10, 10));
The sprite is a rectangle with a height and width of 10, and whose leftmost pixel is at column 45 of the image from the left and whose topmost pixel is at row 45 of the image from the top.