If you just want to retrieve the width/height of your sprite, I suggest to use the texture rect instead of the global bounding box (see Laurent's comment about transformations):
float width = sprite.getTextureRect().width;
float height = sprite.getTextureRect().height;
This code is much faster but it won't work if you have applied transformations on your sprite (such as rotation, scale, etc.).
If you only care about the scale transformation, you can still get the correct size without using the global bounding box:
float width = sprite.getTextureRect().width * sprite.getScale().x;
float height = sprite.getTextureRect().height * sprite.getScale().y;
(By the way, this is the exact implementation of sf::Sprite::GetSize in SFML 1.6).
Pick the solution which best fits your needs.