Don't know your A5s from your DLs? Here's my essential guide to UK paper sizes...
The most common system of paper sizes in Europe is the ISO standard. Most people are familiar with the A series which includes A4 (the size of a standard letterhead):
A0 - 841 x 1189mm
A1 - 594 x 841mm
A2 - 420 x 594mm
A3 - 297 x 420mm
A4 - 210 x 297mm
A5 - 148 x 210mm
A6 - 105 x 148mm
Another popular size for printed leaflets is 1/3 A4 (a third of A4, sometimes referred to by the size of its associated envelope - 'DL') which is the size of a standard compliments slip at 210x99mm.
A promotional mailer or leaflet can have extra impact if it's produced at a non-standard size. If you plan to head down this route, there are a few issues you should be aware of: