SDXL native trained resolution for 1:1 aspect ratio is 1024x1024 like SD 1.5’s is 512x512. Like SD 1.5, you can go a bit below or above that without too much problem; unlike SD 1.5, SDXL also has significant training in a fairly wide set of other resolutions (ranging from 2048x512 to 512x2048) with approximately 1 mebipixel resolution, and they can be treated as starting points as easily as 1024x1024 can. I think SDXL has a narrower (proportionate) range of viable resolutions around its starting points, but that’s offset but having more than one “starting point”.