You can easy find some SMD components which are pretty small for such a high voltage (500V @ 0805 package for example - crazy?!). 0805 case would have 1.3 mm distance between the contacts, so I doubt that this voltage would be normal under real world circumstances.
So my questions are:
Would you pick such component for your circuit? I'd better place two or even three components in series to make the voltage across 0805 case not more than 100-150 Volts or 150-200 for 1206. But this estimation based on my sensation (I never experienced any failures or never researched the limits)
Probably such components should be used inside sealed cases or under a layer of a good insulating varnish (which is rare for me)? This could explain their existence.
Do any of you guys have any experience with these components? What is the real-world V/mm limitation?