In my experience when looking for a tank many aquarium shops will start off with a rough length measurement (metric or imperial here in Au) then get down to more exacting measurements (in metric) later while you are getting more selective.
For example when asked what size tank one might quote "about four foot", but no indication of volume.
Originally I bought a "4 foot" Jebo, and by the time I got it home I still had no idea of the volume and had to calculate it, but as it had a curved front and corners it was not possible to calculate it perfectly, roughly it was about 380 litres. Although it was four foot (converted is 120cm), it was actually 128cm.
My replacement tank (the Jebo cracked after three weeks
), a Juwel, is also a "4 foot" tank with a curved front, is actually 120cm and 260 litres. The Juwel's exact measurements and volume were stated by the manufacturer on the advertising material, and they even have a fill line inside the tank.
From what I saw while shopping for them, there is no standard sized tanks here in Australia, seems everyone makes them to suit the customer or themselves (so there is minimal waste from materials etc).
Off Topic
Measurements here are more dependant on the generation of the user, my mum grew up talking imperial but she has gotten her head around centimetres, my generation was mostly metric although as a kid peoples height was still in imperial. As a tradesman I had to correctly learn the "old scale" so I know that 40/1000 (40 thousandths) of an inch is 1mm. My wife is entirely metric, so when she asked me my height and I said "5'11", her response was ...
Then I confuse her more by saying that there is 12 inches to a foot, and 3 feet to a yard...