Nelson's column? Would even be appropruately navalish.
Good point. Now we've all re-watched John Noakes's ascent, most of us even have a vague idea how tall it is.
There's also the thickness of a human hair, though that's even less useful for measuring aircraft carriers than the size of Wales...
The football pitch comparison is completely acceptable IMO, I can't see the problem meself. Even football widows have watched the kids play, so can visualise based on that.
Kids all seem to play 5 a side these days, so even the humble football pitch is open to misinterpretation.