You are right! To quote the HyperTalk 2.0 reference by Dan Winkler and Scot Kamins: "Locations 0,0 through 511,341 are within a standard card's area and are therefore visible on all Macintosh displays. Points outside those boundaries might or might not be visible, depending upon the size of the display you're using." Page 872 defines the Standard card pixel size as 512 horizontal, 342 vertical. HyperCard 2.0 allowed already to make custom sizes for stacks and my screen at that time was 640x480.

Hmm, could be. I first used HyperCard on a Mac IIfx and I cannot remember to have it used on my Mac SE/30 before. Will check - the original HyperCard manual from Atkinson is still somewhere in my shelves.