The IWF standard is indeed 450mm for all bumpers 10kg and above. All comp and legitimate training bumpers will also conform to this. In regards to techniques, I haven't seen a lot of them, but are for the most part in the 440-450mm range. 430 is a bit small and not "full size."
5kg, are you looking at purchasing or just wondering?
Edit: Keep in mind the true purpose is to be able to practice technique with very little weight AND be able to drop the bar. Bumper plates below 10kg just do not have enough mass to withstand it's own weight and the bar. Dropping a bar with a 5-7kg bumper will not last more than a few drops, depending on height. Plus there aren't many bumpers that go below 10kg - in WL comps, weight below 10kg is all iron/rubber incased iron at the regular "iron" size.
Last edited by Pull14; 09-24-2011 at 04:00 PM.