There are many different kinds of leaflet page size formats that you can use for your leaflet designs. What leaflet size you go for depends on how you plan to distribute or display your leaflet and how much content you have really and what space you need to most effectively communicate your leaflet content. The most common leaflet sizes in the UK are based on the standard ‘A’ series, so A6, A5, A4 etc etc and also the DL size (1/3 A4) .
My most popular leaflet size is the A5 format (and usually double-sided to make the most of both sides) The A5 leaflet works well to either hand-out in person, post through a letterbox or display in a dispenser. For clients who need more space I recommend a 4 page A5 leaflet which is basically an A4 leaflet folded in half. See below for the specific dimensions of all popular paper sizes and in particular the popular leaflet sizes of A5, A6 and Dl (1/3 A4) :