refs #59: add dialog size

lib/country_code_picker.dart

@@ -22,6 +22,9 @@ class CountryCodePicker extends StatefulWidget {
   final bool enabled;
   final TextOverflow textOverflow;
 
+  /// the size of the selection dialog
+  final Size dialogSize;
+
   /// used to customize the country list
   final List<String> customList;
 
@@ -74,6 +77,7 @@ class CountryCodePicker extends StatefulWidget {
     this.textOverflow = TextOverflow.ellipsis,
     this.comparator,
     this.customList,
+    this.dialogSize,
   });
 
   @override
@@ -213,6 +217,7 @@ class _CountryCodePickerState extends State<CountryCodePicker> {
         searchStyle: widget.searchStyle,
         showFlag: widget.showFlag || (widget.showFlagDialog == true),
         flagWidth: widget.flagWidth,
+        size: widget.dialogSize,
       ),
     ).then((e) {
       if (e != null) {

lib/selection_dialog.dart

@@ -10,6 +10,7 @@ class SelectionDialog extends StatefulWidget {
   final WidgetBuilder emptySearchBuilder;
   final bool showFlag;
   final double flagWidth;
+  final Size size;
 
   /// elements passed as favorite
   final List<CountryCode> favoriteElements;
@@ -24,6 +25,7 @@ class SelectionDialog extends StatefulWidget {
     this.searchStyle,
     this.showFlag,
     this.flagWidth = 32,
+    this.size,
   })  : assert(searchDecoration != null, 'searchDecoration must not be null!'),
         this.searchDecoration =
             searchDecoration.copyWith(prefixIcon: Icon(Icons.search)),
@@ -64,8 +66,9 @@ class _SelectionDialogState extends State<SelectionDialog> {
         ),
         children: [
           Container(
-            width: MediaQuery.of(context).size.width,
+            width: widget.size?.width ?? MediaQuery.of(context).size.width,
-            height: MediaQuery.of(context).size.height * 0.7,
+            height:
+                widget.size?.height ?? MediaQuery.of(context).size.height * 0.7,
             child: ListView(
               children: [
                 widget.favoriteElements.isEmpty