I had similar problem, but for some reason autoSize() was not working for me, so I started digging and I wrote workaround for resizing modal window height after it is shown, using jQuery.
var dlg = SP.UI.ModalDialog.get_childDialog();
var header = $(dlg.$I_0)
var userRoot = $(dlg.$L_0);
var frameContainer = $(dlg.$1Z_0);
var headerContentHeight = header.height();
var headerMargin = header.cssUnit("margin")[0] * 2;
var headerPadding = header.cssUnit("padding")[0] * 2;
var headerHeight = headerContentHeight + headerMargin + headerPadding;
var frameContainerPadding = Math.ceil(($(dlg.$7_0).height() - userRoot.height() - headerHeight) / 2);
//frame container
$(dlg.$1Z_0).height(height - headerHeight - frameContainerPadding);
//background white space
$(dlg.$4_0).height(height);
//border
$(dlg.$1H_0).height(height);
//root of user html
userRoot.height(height - headerHeight - frameContainerPadding);
//frame
$(dlg.$7_0).height(height);
//center the window
$('.ms-dlgContent').css('top', $(window).scrollTop() + ((($(window).height() - $('.ms-dlgContent').height()) / 2) - 30) + "px");
Keep in mind that the actual height of the window is larger than the the one in the dialog options. This function will set the actual height of the dialog window to match the new height.
Maybe this solution could help anyone else, having this problem. This is all modifying the styles, so it should not be a problem, but it is kind of a "hack" so it could be buggy.