Resizing in the image editor after a sprite image has been saved once doesn't change the "Size" of the sprite, i.e.
1) add sprite,
2) load image in image editor,
3) exit and save image, sprite "Size" is set to image's size,
4) go back in image editor and crop transparent edges or scale image,
4) save and exit, sprite "Size" doesn't update and sprite is stretched back to original size.
[Personally I'm here because the HTML5 focus is so ahead of the curve compared to other non-programmer game-making software. Everyone else is flirting with it, but seems to have too much invested in flash or whatever to make such a drastic change.]