Nano Banana 2.1 21:9 ultrawide: 1584x672, 3168x1344, 6336x2688

Nano Banana 2.1 makes a 21:9 image at 1584x672 (1K), 3168x1344 (2K) or 6336x2688 (4K), per Google's size table. To fill a 2560x1080 ultrawide banner, take the 2K frame, scale it by 0.808 and trim about 6 px of height.

Sume forwards your ratio and tier to the provider and does not document the exact pixels it returns, so this post uses Google's table. Check width and height on the first file you get back before you build a crop on them.

What are the 21:9 sizes?

The frame is 2.357 to 1, a touch narrower than the 2.370 of a 2560x1080 screen, which is why the trim is on height and not width.

How do I reach 2560x1080?

Scale 3168x1344 by 0.8081, which makes the width exactly 2560 and the height 1086. Trim 6 px of height, 3 from top and bottom, and you have 2560x1080. The 1K frame needs a 1.6 times upscale, so it is only good for a layout proof.

If you need 3440x1440, start from 2K as well: the scale factor is 1.086, a small upscale that a normal resampler handles cleanly.

• Generate at 2K, not 1K, for any banner you will publish.
• Crop after scaling, not before, so the resampler sees the whole frame.
• Sume never applies target_pixels; the scale and crop run on your side.

Which Sume models list 21:9?

On Sume, Nano Banana 2.1 lists 21:9 but not 9:21. The FLUX.2 Pro and Flex rows, Qwen Image and Recraft V4 list both 21:9 and 9:21, and Ideogram 4.5 lists neither. Read GET /v1/images/models for the row you plan to call.

{
  "model": "google/nano-banana-2.1",
  "prompt": "Wide desert road at dusk, no text",
  "aspect_ratio": "21:9",
  "resolution": "2K"
}

What does it cost on Sume?

The 2K billed price is $0.15 per image, all-or-nothing. Rows and limits are in the image models docs.

Wide frames reward a prompt that says what fills the width. Name a foreground, a midground and a background, or the model will park one subject in the middle and leave the sides empty. If the banner is going to carry text, leave a plain area for it in the prompt and add the type in your layout tool rather than in the picture.