Varies a bit on old existing infrastructure. But 240mm is widely used, sometimes copper and sometimes aluminium. Some DNOs use copper from the substation to the first joint, and then aluminium thereafter. Copper gives a greater margin for de-rating when a number of cables are grouped together near the substation. Aluminium is cheaper.

Service cables also vary a bit, but 25mm copper or 35mm aluminium are often used. Sometimes one size up if the service cable is longer than the norm.

Both street mains and service cables are constrained by voltage drop rather than by current carrying capacity.

Thanks Broadgage ì asked partly cos when I had to have a smart meter recently they upgraded the tails from 16 to 25 mm  which got me thinking what size is the DNOs main now I know  also someone reckons there main is only 4mm that seems  somewhat unlikely 

I have seen old service cables in 4mm, or more likely an old imperial size similar to 4mm.

Sometimes they go bang, but many survive. Back in the good old days, household services were often only 20 or 30 amps. Who could want more than that ! I have seen old electricity meters designed for 10 amps at 110 volts DC. Ample for lighting and a radio set. Not many houses contained more than a dozen lamps. And whilst the odd 100 watt lamp might have been used, the average lamp wattage was probably about 40 to 60 watts.

Kitchen----------------one lamp of 100 watts.

Living room-----------one lamp of 100 watts, or in the better class home, an "electrolier" with four or five 40 watt lamps.

Stairs/hall/landing----three lamps each 25 watts.

3 bedrooms------------each one 40 watt lamp

Bathroom--------------two lamps each of 40 watts.

WC----------------------25 watts.

A 10 amp service at 110 volts would be fine for that lot. One could probably get away with a 700 watt electric kettle or a 700 watt clothes iron as well as the lights. Service cable was probably about 2.5mm There were still a very few 110 volt DC services into my lifetime. 

new 100 A services are likely to present in 35mm from the street to the house, the street main is fatter. This is not so much a cable current rating thing , but to ensure the substation fuse pops before the house supply cable, at least for a dead short.

Older stuff, who knows. not always the DNOs even.  But the older lead clad oily paper sort of cable was better cooled and could take excursions to well over 100C without failure, so do not compare just core area with a modern PVC or XPLE insulation.

Even so 4mm2 is probably a bit mean, but something between 6 and 10 is credible, and probably OK on a 60A service fuse as often in a 100A holder.

DNOs vary a bit but not much, this is UKPNs for new connections from EDS 08-2100

available here UKPN library  You do have to create an account to access this, but it is only an Email and there is a lot of other useful stuff. . You can always put IET forum user in the 'job description'  if you wish to be coy.

Mike.