A0 = 841mm x 1189mm 16 x A4 Sheets

A1 = 594mm x 841mm 8 x A4 Sheets

A2 = 420mm x 594mm 4 x A4 sheets

A3 = 297mm x 420mm 2 x A4 sheets

A4 = 210mm x 297mm 1 x A4 sheet

A5 = 148mm x 210mm ½ of an A4 sheet

A6 = 105mm x 148mm ¼ of an A4 sheet

If using PowerPoint to create your poster,  always set you page size before you start, (Click design, Select page sizes, enter page sizes in CM not MM)