Yeah, that's the problem though. How do I get the exact size?4.5mm allen keys are available but it's worth getting the exact size before trying otherwise it will only damage the nut and eventually the only thing that will budge them would be a snap on stud extractor!
I found something via Google of someone saying 'either 4.0, 4.5, or 5.0mm' depending on which country. It then looked like China was 5.0 and my bass supposedly is China build. 5.0mm is definitely too large though, and so I didn't force it.