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>

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>

你可以用百分比表示宽度。

<style name="Theme.Holo.Dialog.MinWidth">
<item name="android:windowMinWidthMajor">70%</item>

在这个例子中，我使用了Holo Theme。

设置自定义对话框布局的父布局为RelativeLayout，自动获取公共宽度和高度。

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

@Override
public void onStart() {
    super.onStart();
    Dialog dialog = getDialog();
    if (dialog != null)
    {
        dialog.getWindow().setLayout(-1, -2);
        dialog.getWindow().getAttributes().windowAnimations = R.style.DialogAnimation;
        Window window = getDialog().getWindow();
        WindowManager.LayoutParams params = window.getAttributes();
        params.dimAmount = 1.0f;
        window.setAttributes(params);
        window.setBackgroundDrawableResource(android.R.color.transparent);
    }
}

当我需要使对话片段更宽一点时，我设置minWidth:

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:minWidth="320dp"
    ... />