Extra space below the message is important, so you have a place to put your hands.
Here are some poster templates! Both of them are for 11×17 paper. They should adjust to A3 size pretty easily.
The way you use these is to open the link below, and then go to File -> Download as -> and probably pick PowerPoint (pptx). Alternatively, you can add a copy to your own Google Drive and mess around with them in Google Sheets.
These are intentionally simple. Copy them, alter them, make them your own. Play with the font and the images. If you do export to PowerPoint, you may find that the margins have disappeared. Some font adjustment will be necessary.
Icons in these templates are from http://game-icons.net/, which has a ton of icons that you can recolor right on the site and download in PNG and SVG format.
Enjoy! More to come.