Powerpoint is a very easy software to use as the Microsoft interface is familiar and easy to navigate. Microsoft has many great templates for slides that can be used for posters.
When using powerpoint, be sure to set the slide size to the actual size of the printable poster. This way you avoid images or text that is stretched out or pixelated.
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".