We used poster size A2 for our designs. You can either rebuilt the entire design if you want to use a different poster size, or you can upgrade to Canva Pro (30 free trial) and use the amazing Resize Tool.
Canva has hundreds of thousands of free to use photos and graphics available. But 25 times (!!) more photos and graphics are available exclusively on Canva Pro which makes Canva Pro a fantastic deal if you have lots of designs to make.