I want to design a safety circuit for a battery powered device which measures human body impedance. It is intended to be powered only by an external battery pack (5V) via a USB cable. But I want to make sure that even if device is connected to wrong power supply by mistake (e.g. a faulty USB charger leaking 230V), the user is safe. I am also limited in PCB size. What kind of circuit or IC should I look for this purpose? I couldn't find any regulators that can tolerate such high input voltage.
You need to study isolation. If you put a USB connector on the device, you'll need two means of patient protection, 5kV of isolation, and 250V working voltage to be medically safe. There are isolated DC-DC modules available. All of this is contained in IEC-60601. Study it. People are killed by designers that don't follow the rules.
To make this stand up to mains voltage on Vcc seems a bit misguided (as opposed to making sure it's never plugged into mains), but here's how I would approach it-- as @BrianDrummond suggests.
If that's not realistic for your case, I recommend completely isolating the stimulus and receiver by using transformers, and diode or zener-clamping both the inputs and outputs to the transformers. The concern is that if you have 230V AC on these diodes, they'll fry up and stop doing their job. I'd try to look for diodes that can hold up to that, but I'm not sure you'll have luck.
Thus, to be double safe I'd probably use an isolated DC-DC converter to generate Vcc, paying attention to the likely failure modes of the device you use. Mains would be unlikely to punch through. Hopefully, there'd be no way for the 230V AC to find its way to the isolated side, but the research on that would be up to you. Even if the mains can poke through for a little while, the idea is to have the diodes last long enough for the converter to fail.
Frankly, the easier way to do this is to just use an IEC 60601 compliant power supply or just build in a battery holder so there's no other way to plug in the wrong supply.
One approach against casual mis-connection is to eliminate the USB cable, and to use some other connector style as the connection from battery pack to equipment.
You can use a separate USB connector on the battery pack for recharging purposes, and a 2-pole mechanical "break before make" switch set to "charge" or "use" connecting the battery to either charging circuit or output. Then isolation can be guaranteed up to the voltage rating of the switch.
There are other approaches, but this has the benefits of simplicity and obviousness in its operation, which will help if you must make a safety case for it.
I think I would consider a lithium battery power supply and designing a mechanical interlock of some kind so that the unit cannot possibly be used while attached to the charger (perhaps the connectors for the probes physically cover a micro-USB port).
Also put an electrical interlock in there so even if a determined user defeats the mechanical interlock it can't be used while it is charging. And read the standards, make sure you're 100% compliant with safety standards and current best practices, and maybe then talk to a lawyer about product liability.
I would definitely treat the USB cable as a mains connection if it has any kind of connection to a human being that is different from casual contact, and especially if it's a medical-ish device.
This is a place where you could very very easily kill a patient. Install a battery holder, use the lowest-voltage battery that can possibly satisfy the circuit, build for the highest input impedance you can tolerate, and DO NOT provide any means of connecting the device directly to USB. If data transfer via USB is a requirement, use optoisolators.