Ha ha, bad luck. You'll thank your lucky stars in time - white stems look cool for a short while then you start to wonder why you didn't stick with black. I couldn't say for certain but I think they are 25.4mm handlebars which was once the standard I believe. I have no idea why Giant used these sized ones instead of 31.8, nor why their seatposts are 30.9mm in diameter, but it does make sourcing parts a bit challenging!
On the original issue, it might be worth double checking that both the top and bottom bearing setups on the fork/headtube are all present and correct... Because it *could* theoretically be misaligned or missing parts in the bearing setups that made the fork steerer too long. If so, then simply adding a spacer would not solve the inherent problem I don't think. Just a thought.
Stu