The 5 pound sterling banknote measures approx. 13.5 cm long by 7 cm wide
Here is a list of the euro bill dimensions
About 5.3 inches x 2.7 inches. I know we're meant to be metric here but most people are hybrid. My age range certainly tend to use metric weights, and old imperial measures.
Anyway more on topic, those are measures for new notes, once they've been creased up and used they tend to shrink somewhat.
Best off grab a tape measure
Also some tips on how to do it
youtube, clean link
--edit (08/08/11 2300 BST)--
Well the results are already looking interesting so far, baring in mind the average penis size according to professional studies is around 5 inches* fully erect. We here clearly have some big boys here. The modal average for us big lads is 7 inches, a good two inches above what you'd expect. In fact only 7 people claim to have an average or slightly below average sized member, while a 12 range from slightly above average through to a size enjoyed by 0.1% of people!
(*I know the largest study showed an average of around 6 inches, but it was self measure, which to me rather confounds the results. Two subsequent studies, all be it with smaller sample groups have shown average sizes of 5.1 and 5.7 inches)