title: How to find the widget's current size to set the minimum size for the constraints in the Flutter?
body: <p>I want to know how can I find a widget's default size to set it as <code>constraints: minHeight</code> or <code> constraints: maxHeight</code> properties?</p>
  <p>For example if I have this widget:</p>
  <pre><code>      Expanded(
          child: Column(
            children: [
              // right menu 1
              Expanded(
                child: Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: Container(
                    constraints: BoxConstraints(
                      maxHeight: constraints.maxHeight - 16,
                      minHeight: 100,
                    ),
                    decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(8),
                      color: Colors.blue[400],
                    ),
                  ),
                ),
              ),
              // right menu 2
              Expanded(
                child: Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: Container(
                    constraints: BoxConstraints(
                      maxHeight: constraints.maxHeight - 16,
                      minHeight: 100,
                    ),
                    decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(8),
                      color: Colors.red[200],
                    ),
                  ),
                ),
              ),
            ],
          ),
        ),
  </code></pre>
  <p>And I want to set <code>minHeight</code> to the container's default height instead of hardcoded 100.</p>