I'm about to get to work on a mobile app but I don't know what the best size is.
You can set the layout size to anything, It won't cause any performance issues. If you meant the resolution of the game screen it's worth checking out this link:
https://www.scirra.com/tutorials/73/sup ... reen-sizes
I usually go with 1280 x 720, then apply the Support Multiple Screen Tutorial that tarik00555 provided to my project
Develop games in your browser. Powerful, performant & highly capable.
I usually go for 1136x640 with a 960x640 viewport/'safe zone', and scale outer. It works well for wider aspect ratios like iPad.