When forging an image, you can specify the aspect ratio in which the image is generated. Since Layer supports creating higher-resolution images after initial generation, don’t worry too much about resolution.
The default options for aspect ratio are:
Square: Square image with even width and height. Ideal for generating most assets.
Portrait: Taller image. Ideal for backgrounds / UI for portrait-orientation games
Landscape: Wider image. Ideal for backgrounds / UI for landscape-orientation games
Note that if you use a reference image or sketch, the ratio of your Forges will match your reference.
If you generate something but want to change the aspect ratio, try using the Asset Studio’s canvas tools to extend your art asset.