My head measures 57.15cm.. 57 is the entry level for a medium but I was wondering if a small would be too small or after I break it in will it fit more 'normal'?
I am not sure which model scorpion helmet you are asking about The EXO 400 & 700 small and medium helmets are the same size shell with the only difference being the liner size. I would think the other models are the same, but I have not inquired. So if later you think you made a mistake you can correct it by getting the other size liner. I know this does not answer your question, but it might help you down the road. If possible I would try on a medium and see if it is too large.
I have a 57cm skull and have had both the EXO-700 & a new EXO-400 both in Large, they have a snug fit which I like for riding track, where my ICON & SHoei lids are Medium and are a bit looser and I use on street.
A few years back, I had a track coach (not STT) tell me that you should find the smallest size helmet that fits and then buy the next size SMALLER. One of the things the helmet is supposed to do is keep your brain from sloshing around inside your skull during an impact. If the helmet is at all loose, your head will move inside it when you take a hit and you'll have additional 'sloshability' which is not a good thing. Regarding the Scorpion, my pea-head is just over 56mm around. I wear a small EXO-700. It feels very snug when I put it on and whilst I'm sitting on the grid. But once I'm in motion, I never notice it again. +1 on STG being willing to work with you on getting the correct fit. Hope this helps.