-
They're fairly narrow. I have 8s which is what I usually wear (both sidis and regular shoes are men's shoes though) but they are definitely slightly too small. If buying again I'd go with 8.5s. 8 mega (wide fit) might be ok.
Additionally if you have flat feet and planning on using a different insole it's worth knowing the standard sidi one is quite thin, so swapping it out affects sizing.
I think it probably depends on the shape/width of your feet more than anything. I wear the same size as my street shoes (41) but I think I'm in the minority there. Best thing is to try some on I reckon. I know Sidi's don't suit everyone.