Demand Gen square 1200x1200: image vs logo file limits

Request image_size: "1200x1200" from a GPT Image 2.5 model on POST /v1/images. The Demand Gen spec Google publishes, read 2026-09-30, lists 1:1 images at "Min. size: 300x300, Recommended: 1200x1200", and 1200 is 75 x 16, so it passes Sume's custom-size rule.

Which sizes does the spec list for the square?

Google lists a square logo and a square image with the same recommended size but different limits, so check each against its own row.

How do I request it?

1200x1200 is 1,440,000 pixels at 1:1, inside the custom-size box in the image docs. If a model does not accept custom pixels, 1:1 is also a native aspect_ratio value, and the request body is the same as for any other size: model, prompt, image_size: "1200x1200" and an Idempotency-Key header.

Which output format should I ask for?

Google's snapshot says .jpg or .png for both assets, and the Sume Image 1.0 page lists png, jpeg, jpg and webp. Skip webp here. A 1200x1200 PNG can easily pass 150 KB, so for a logo prefer jpeg and measure the returned file; the cap is Google's, and the API has no size-cap parameter in the docs read.

How do I keep the logo under its cap?

Treat the two assets as two requests. The image can be a photo; the logo should be flat artwork with few colors, which compresses far smaller. Generate it, download it, and check the byte count against Google's 150 KB before you upload. If it is over, ask again with a simpler prompt rather than resizing, because the pixel size is already the recommended one.

What if I need the other ratios too?

Square is the simplest case. For the 4:5 size see Demand Gen 4:5 image size 960x1200; the full set is in one reference, four ratios.