Oscar,

The shot placement guides recommend the .375 H&H or 9,3mm but also state that a .300 magnum or .338 magnum can be used.
When I saw the giraffe; and when I saw the thickness of the skin and the sheer mass of the shoulders I became convinced that the sensible minimum is the 9,3mm. It is such a big animal, though, that I don't think you can be really over-gunned (although the giraffe is not dangerous game).
If you don't shoot solids, then make sure the bullets are premium.
The guys in Zim seem to be very impressed by the Barnes X bullets, and I shot mine using the Swift A-Frame.

- Lars/Finland