How to set image width or height in gemini-image-2.5-flash API?

I checked the official Gemini API documentation and only found the aspectRatio parameter—I couldn’t find any options to directly set the image width and height. Is there a way to specify the output image width after setting the aspect ratio?

I’d like to generate images at 512 × 512, which is sufficient for my needs. Generating larger images just consumes more tokens.