How big is that?

Neville Goodman

If you want to convey something's size, try a familiar comparison. There was a story about a hapless biology teacher who had reached the birds and bees lesson. Faced with a class of pubescent girls and needing to tell them how big the testicles were, she ventured that they were about the size of a peewit's egg. ‘Well, now we know how big peewits' eggs are,’ came a stage whisper from the back. Doctors usually go for fruit. Tumours are the size of almonds, grapes, tangerines, oranges or grapefruit. These sizes are a bit inexact: would those be the small delicious green seedless grapes, or the big black seeded grapes with tough bitter skins?

We also like sports fields, comparing the alveolar surface area to a football pitch, and the total surface area of the nephrons to three cricket pitches. I have no idea if these are even vaguely correct, but you get the idea.

It's not just in medicine that we need comparisons. How else can anyone convey how much rain forest is lost each year or how much of the UK has turned into car parks since Labour came into power? Most land areas used for comparison are, like the Isle of Wight (380 sq km) or Belgium (30 500 sq km), roughly rectangular. This probably explains why Wales, which at 20 800 sq km is a bit less than a Belgium, is a standard unit of measurement, whereas England and Scotland are almost never used. England has too many awkward bits, and Scotland's islands make it useless.

Frankly, in the scale of these things, there isn't much difference between 20 000 and 30 000, and I bet there are a fair number in Britain who don't have a clue how big Belgium is, so we might just as well stick with Wales. The next order of magnitude takes us to France, which is 547 000 sq km. The US has enough states for quite a range of units. For all I know, Americans use the full range, but there's little point our measuring things in Marylands (25 000 sq km) instead of Waleses. The Texas is universal currency; at 695 000 sq km it is considerably bigger than France.

So how about a comparison I saw for the Venus Express, a European spacecraft launched in November. It is apparently ‘little bigger than a fridge’. So once you've decided what sort of fridge — dinky job for a student bedsit or enormous American thing with double doors that spits ice cubes — it's then a bit bigger. An ovarian tumour the size of a melon is much more imaginable.