Photoshop's Generative Upscale (Image > Generative Upscale, powered by the Firefly Upscaler) enlarges images 2x/4x and fills in missing detail via AI rather than just stretching pixels, producing markedly better results than a plain non-AI resize.
Hard ceiling: Output cannot exceed 6144px on the long edge. Attempting 2x on a 4,000x6,000 source (would be 8,000x12,000) throws 'output is too large.'
Workaround: Shrink the source first (Image > Image Size) so the scaled result lands under 6144px — e.g. downsize to ~1,000x1,500px before a 4x pass. Ideal starting long edge is roughly 2,000-4,000px (smaller also works); back-calculate from the 6144px cap against the desired scale factor.
Caveat: The downsize-then-upscale trick was demonstrated only to illustrate the pixel limit, not recommended as a real workflow — don't shrink a large image just to blow it back up. The genuine use case is upscaling small originals, e.g. scanned/photographed old family photos.
Partner models: Not yet available for Generative Upscale, unlike Generative Fill, which supports partner models like Gemini 2.5 Nano Banana.