How do I resize a movieClip?

I understand that this is something really, really basic, but for all my Googling I can’t seem to find a clear answer.

I have a movieClip with around 1500px of vertical content inside (a grid of images). But when I trace its height, it says it’s only 400px high. I need it to actually report a height of 1500px, or it doesn’t scroll properly.

But this doesn’t work:

content.height = 1500;

It stretches out the content. And this doesn’t work:

content.height = 1500;
content.scaleX = content.scaleY;

It just makes the content HUUUUUUUGE. And, I mean, I understand that in Flash setting a width and height messes with the scale, so this should be entirely expected. But there has to be a way to set the height of a movieClip.

One suggestion I found was to add another movieClip or sprite inside of the movieClip you’re trying to set the height of. I added another movieClip with a height of 1500px, but it didn’t have any affect:

var sizer:Sprite = new Sprite();
sizer.height = 1500;
content.addChild(sizer);

I still get around 400px.

So, what’s the correct way of doing this? There’s no way it could be this hard to do something as simple as set a height. What am I missing?