Auto-size Ext JS Window based on content, up to maxHeight

Using Ext JS 4.0.2, I'm trying to open a window that automatically sizes itself big enough to fit its content, until it hits a height limit, at which point it stops getting bigger and shows a scroll bar.

Here's what I'm doing

Ext.create('widget.window', {
    maxHeight: 300,
    width: 250,
    html: someReallyBigContent,
    autoScroll: true,
    autoShow: true
});

When the window is first rendered, it's sized big enough for the really big content--bigger than the maxHeight should allow. If I attempt to resize it, then snaps down to the maxHeight of 300px.

How do I constrain the window to its maxHeight when it's initially rendered?