Posters and leaflets are very important marketing collateral. The ususal sizes are A4, A3, A2 and A1, depending on budget and final poster size. However there are also specialist sizes such as 'Double Royale' etc which are used in bus shelters and so on.
Leaflets also can be designed in a variety of sizes from 1/3 A4 (DL), A6, A5, A4, A4 folded to A5, A3 folded to A4.
As an experienced poster designer and leaflet designer, based in Hanwell, West London, I'm able to create any size and design for you. Please call me to discuss your requirements, or I have access to good transport links and can meet you.