From memory [but it's easy to check] copper hot water tanks are normally 450 diameter [uninsulated diameter].... so a 140 litre capacity [ suitable for a 2 bed house] will be about 900 high, a 210 litre one about 1300 high or so [suitable for a 3 or 4 bed house...
if you replace it get one as big as you can and make sure it has 100mm foam insulation all around it and insulate any heating and supply pipes with 50mm foam pipe insulation at least 2 heating coils in it so you can have a primary central heating hot water source and then you can fit a solar hot water circuit later and a twin electric immersion heater in it for emergencies.
They are generally standard and so if you can measure the height of it then you can look at an on line plumbers shop site and see the capacity of the same size. As above they stay standard sizes so that they can be replaced.
One point to note is hard water.
I have a normal tank but over about 10 years it fills up with scale that builds up and drops off the coil and element and gradually fills up the tank with bits. In 10 years it ends up at least a quarter full. If you live in a hard water area this may be happening and this will cut down the capacity of the existing tank so you can get better capacity with just a new tank of the same size that is not full of scale.