By far the easiest program for creating posters used to be "Textease" produced by software company Softease, and I used it in several primary schools for over ten years. There was also a Home version that parents could buy.
It was a bit like a 'painting' program, and you could click ANYWHERE on the page to type items of text, or insert pictures or create drawings.
Items could then be manipulate by dragging them, could be re-sized, copied, rotated, stretched or squeezed using the mouse, sometimes in conjunction with Shift or Ctrl keys. And the text could still be edited, even after manipulation; so if you found a spelling mistake, or changed your mind on a word or date, you could still correct it.
Multi-pages could be automatically printed, to make GIANT posters, and 'crop marks' showed how to paste the A4 pages together: I forget what the largest I did was, but probably twelve or sixteen pages.
HOWEVER: I have been looking, and cannot find a reasonably priced edition, and it now appears only to be 'bundled' with several other educational programs - database, spreadsheet, presenter, and others.
I'll give you a link, which has email details, and you could probably find a phone number. You could ask if there is still a 'home' version, and it would certainly allow you to create the best posters you have ever seen!
www.segfl.org.uk/index.php/suppliers/show/softease/3273/s
A Professional program is Xara. This would be expensive, but there might possibly be a demonstration version you could try for a limited period of time. Have a look at Xara Gallery, to see what CAN be achieved!
www.xara.com/uk/photo-graphic-designer/