Jam jars in two sizes. One holds 230ml and has a diameter of 66mm and the larger holds 440ml with a diameter of 82mm. Both have straight sides - no tapering at the neck of the jar either.
According to the BBC website this is how you sterilise jam jars:
Wash your jars and the lids in hot soapy water, but do not dry them. And importantly, this just in:
"As a WI jam maker, please may I ask you to add the instruction to rinse jars and lids in clean water before sterilising them."
Now leave them to stand upside down on a roasting tray while they're still wet. Pop the tray of clean, wet jars and lids in to a preheated oven at 160-180ºC for about 15 mins.