Working on Android 6.0, ran into the same issue. AlertDialog would default to predefined width set in the theme regardless of the actual width set in the custom view's root Layout. I was able to get it to set properly adjusting the width of the loading_message TextView. Without investigating further, it seems that sizing the actual elements and having the root Layout wrap around them makes it work as expected. Below is an XML layout of a loading dialog which sets width of the the dialog correctly. Using the this library for the animation.

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@color/custom_color"
    android:padding="@dimen/custom_dimen">
    <com.github.rahatarmanahmed.cpv.CircularProgressView
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:id="@+id/progress_view"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:layout_centerHorizontal="true"
        app:cpv_color="@color/white"
        app:cpv_animAutostart="true"
        app:cpv_indeterminate="true" />
    <TextView
        android:id="@+id/loading_message"
        android:layout_width="100dp"
        android:layout_height="wrap_content"
        android:layout_below="@+id/progress_view"
        android:layout_centerHorizontal="true"
        android:gravity="center"
        android:textSize="18dp"
        android:layout_marginTop="@dimen/custom_dimen"
        android:textColor="@color/white"
        android:text="@string/custom_string"/>
</RelativeLayout>

我修复了它设置根元素布局参数。

int width = activity.getResources().getDisplayMetrics().widthPixels;
int height = activity.getResources().getDisplayMetrics().heightPixels;
content.setLayoutParams(new LinearLayout.LayoutParams(width, height));

其他的答案都对我没用。对我来说，解决这个问题的方法是创建一种样式，你可以选择你想要对话框占据屏幕的百分比:

<style name="RelativeDialog" parent="android:style/Theme.Dialog">
    <item name="android:windowBackground">@android:color/transparent</item>
    <item name="android:windowNoTitle">true</item>
    <item name="android:windowIsFloating">true</item>
    <item name="windowNoTitle">true</item>
    <item name="windowActionBar">false</item>
    <item name="windowFixedWidthMajor">90%</item>
    <item name="windowFixedWidthMinor">90%</item>
    <item name="android:windowMinWidthMajor">90%</item>
    <item name="android:windowMinWidthMinor">90%</item>
    <item name="android:colorBackgroundCacheHint">@null</item>
    <item name="android:windowIsTranslucent">true</item>
    <item name="android:windowAnimationStyle">@android:style/Animation</item>
</style>

而不是像这样设置对话框的样式:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setStyle(STYLE_NO_TITLE, R.style.RelativeDialog)
}

使用RelativeLayout作为DialogFragment的父元素

最外层布局中的维度在对话框中不起作用。您可以添加一个布局，其中设置的尺寸低于最外层。

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content">

<LinearLayout
    android:layout_width="xxdp"
    android:layout_height="xxdp"
    android:orientation="vertical">

</LinearLayout>

这将会非常有效。

@Override
public void onResume() {
    super.onResume();
    Window window = getDialog().getWindow();
    if(window == null) return;
    WindowManager.LayoutParams params = window.getAttributes();
    params.width = 400;
    params.height = 400;
    window.setAttributes(params);
}