Generative AI vs Image Size

So, what am I doing wrong here?  I'm developing a game and using generative AI for various art assets, which it is fantastic at... however I've run into a simple yet annoying issue.   

I want to make a button graphic.

I create a new image at 512 x 128.  

I tell AI "A stale colored rectangluar button"

It generates 3 fantastic buttons, however, none of them are 512x128.  They are larger, and get cropped.  The generative preview shows the full button size, but the image itself is cropped.

So what do I do if I actually need a 512x128 image?   Am I setting it up wrong?  Is this just an issue with the AI and getting the size right?

I'm not an artist, I'm a programmer, so my approach could be completely wrong as well.

Thanks for advice!