[Someone]

I think what killed Hypercard, in some sense, was what it made popular: like the Amiga, it was too closely tied to its hardware.

HyperCard ran black and white windows of 512*342 pixels because that is what the original Mac had (there was a version that did some color things, but that was and felt like a serious bolt-on job). Moreover, all graphics where bitmaps.

Bringing that into the 'real world' as it existed at the end of the eighties would have taken a lot of resources, and it was not guaranteed that the end result would still be HyperCard. For example, if one allowed resizable windows, laymen would have to learn about layout algorithms ('is this button 50 pixels wide, or a quarter screen wide? Is it 20 pixels from the bottom, 10% of screen height, or should its bottom border match that of that text field over there?').

Also, at the time, there were attempts to include some Hypercard/Director-like features to QuickTime. HyperCard was considered for that functionality, but did not make the cut.

[ayuvar]

I think that versions of Hypercard past v1.0 allowed for stacks of arbitrary window size; I know for a fact that I made up at least a few 1024x768 ones in v2.2.

You are correct in that I don't think it supported resizable windows without the use of some XCMD though.

[Someone]

You are correct. Surprisingly, I even found evidence for that: http://support.apple.com/kb/TA31048:

* All the cards in a given stack must be the same size, even if there are several different backgrounds in the stack.

* Smallest card size: 64 x 64 pixels

* Largest card size: 1280 x 1280 pixels.