title: How to Size Child of ShowDialog
body: <p>Using <a href="https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/material/dialog.dart#L631" rel="noreferrer"><strong><code>showDialog(...builder: ...)</code></strong></a>, I am <strong><em>unable</strong> to size the content of <code>SomeWidget</code></em>.</p>
  
  <h3>What is happening</h3>
  
  <p>No matter what widgets I try to insert into the <code>builder</code> I <em>cannot <strong>control</strong> sizing</em>.</p>
  
  <pre><code>showDialog(context: context, builder: (context) =&gt; Container(
    color: Colors.white,
    width: 10.0,
    height: 10.0,
  ));
  </code></pre>
  
  <p>No matter which values I insert into <code>width</code> and <code>height</code>, the dialog <em>always displays <strong>full screen</em></strong>. The <code>Container</code> in this case will <em>take up <strong>all available space</em></strong>. The code I posted above will result in the following:</p>
  
  <p><a href="https://i.stack.imgur.com/XCOOo.png" rel="noreferrer">Full size screenshot</a> showing an Android screen, where everything except for the navigation bar, status bar and debug tag is filled out with white. <em>(the <code>SafeArea</code> you can see <a href="https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/material/dialog.dart#L575" rel="noreferrer">comes from here</a>)</em></p>
  
  <h3>Padding</h3>
  
  <p>Doing the same but surrounding it with a <code>Padding</code> and applying <code>padding</code> will <em>work as sizing</em>. Applying padding will remove parts from the side of the <code>Container</code>. This is the <em>only</em> successful way I have come up with. E.g. <code>SizedBox</code> or <code>ConstrainedBox</code> do not make a difference. </p>
  
  <h3>What I would like to see</h3>
  
  <p>Both <a href="https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/cupertino/dialog.dart#L129" rel="noreferrer"><code>CupertinoAlertDialog</code></a> and <a href="https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/material/dialog.dart" rel="noreferrer">Material <code>AlertDialog</code></a> can be inserted as a widget to the <code>builder</code> of <code>showDialog</code> and will <strong>not <em>fill up</strong> the whole screen</em>. </p>
  
  <p>I tried figuring out what those two widgets are doing differently than me, but to me it looks like they are <a href="https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/cupertino/dialog.dart#L94" rel="noreferrer">also using <code>Container</code> sizing</a> or <a href="https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/material/dialog.dart#L80" rel="noreferrer"><code>ConstrainedBox</code> as sizing</a>, which <em>both</em> did <strong>not</strong> work for me.</p>