Standard Design size is a 9:16 aspect ratio.
At it’s largest, the canvas should be: 1080 x 1920 or 1920 x 1080
HOWEVER, it’s good practice in AppyBuilder to design for your device. So if you’re designing for only a phone, you can scale that down to about 30 or 35%.
30% is usually 324 x 576
I made a responsive tutorial that better explains more about screen sizes, if that helps: