Getting the right size poster printed is difficult when you can't see the finished product. Here's a chart that aims to help you visualise how big a product will look once it's printed. Most people will be familiar with the standard A4 size piece of paper, the most commonly used size for business documents, letterheads, schoolwork, brochures and much more.
What about A0? Or A6? The size code gets smaller by 1 number each time the size doubles. So double the size of an A4 sheet to get A3. A2 is double the size of an A3, etc. Go the otherway and you halve the size. Half the size of an A4 is A5. Our cards and postcards are A6 finished size, but our large format posters go right up to A0.