One of the useful features of the new Gemini 3 Pro Image model (aka Nano Banana Pro) is that it can easily create or edit images in a variety of aspect ratios. This is useful because different social media platforms require specific aspect ratios for images. And, of course, they are all different.
Here is a chart from Google’s documentation with all of the aspect ratios that are supported (along with resolution and token information) -
Sometimes you’ll have to prompt it more than once before it gets the aspect ratio right but it should be helpful to know upfront which ones are supported.