Many posters end up with an A0 format. A circumferential margin of 3 to 5 cm is appropriate on an area like this. The poster will look most harmonious when the margin is as wide on the sides as it is at the header and footer ends.

The usable space for the contents of the poster is inside these margins.
Use the guides in your software to define the margins.

Fix the guides so that they do not slip whilst you are creating the poster.