On Mac use Keynote to prepare big posters.

Setup the page size correctly:

First generate an empty page, then go to the inspector. Select the first tab, then click the slide size drop-down and pick “Custom Slide Size”. Keynote assumes 72 pixels per inch, so multiply your desired poster size by 72 and enter that for the slide size height and width. Personally for my poster (44 inch x 36 inch), the numbers are: (3168x2592).

Note that Keynote will insert a white margin around your poster when you go to print it, so you don’t need to put a border around things when designing your poster. You can put objects right up to the very edge of the slide and they will still print OK.