If I use the default "AI Artist" character and ask it to generate images, they are rendered at a resolution of 768x768 (which doesn't appear to be one of the available resolutions as per the documentation) - but in any other character thread, the image command produces only 512x768 images even if I use syntax like (resolution:::768x512) in the prompt. From documentation, the generator should be capable of passing the resolution parameter through to the plugin but either the syntax is wrong or it's not working correctly.
Sorry guys, my mistake. Turns out that characters created by https://perchance.org/ai-character-generator have a default "Image generation prompt ending" set which includes (resolution:::512x768), which is why it was ignoring my own resolution syntax. Hopefully this will help people if they have the same issue - check what's set in that field for your characters!