Typically academic posters are designed on that rule of thirds principles, with some variations of course.
The first poster is a classical example of an academic poster in a landscape format : header with 3 columns underneath. Some academic posters in the landscape format will have 4 columns of information. That’s okay too.
The second poster below is a good example of an academic poster in a portrait format.
There are not hard and fast rules on which layout style that must be used, but consult the conference or event that you are attending to see if they have any prescribed format rules.
Before you add any content to your slide, first adjust your slide to the dimensions for your poster. This ensures that the final product (the printed poster) will look exactly as you designed it and not distorted or blurry.
How to prevent your text boxes from autosizing
When you draw your text boxes on your slide, putting text in those text boxes can change the shape of the textbox. To prevent the text box from changing shape, lock the text box. Right mouse click on your text box, choose "format shape", select "size & properties", open "text box" section and select "Do not Autofit".