refactor: avatar size

lib/screens/commits.dart

@@ -94,7 +94,7 @@ class CommitsScreen extends StatelessWidget {
             child: Row(
               crossAxisAlignment: CrossAxisAlignment.start,
               children: <Widget>[
-                Avatar(url: payload['author']['avatarUrl'], size: 16),
+                Avatar.medium(url: payload['author']['avatarUrl']),
                 SizedBox(width: 8),
                 Expanded(
                   child: Column(

lib/screens/login.dart

@@ -30,7 +30,7 @@ class _LoginScreenState extends State<LoginScreen> {
         ),
         child: Row(
           children: <Widget>[
-            Avatar(url: account.avatarUrl, size: 24),
+            Avatar.large(url: account.avatarUrl),
             Padding(padding: EdgeInsets.only(left: 10)),
             Expanded(
               child: Column(

lib/widgets/avatar.dart

@@ -10,9 +10,25 @@ class Avatar extends StatelessWidget {
 
   Avatar({
     @required this.url,
-    this.size = 18,
     this.login,
+    @required this.size,
   });
+  Avatar.extraSmall({
+    @required this.url,
+    this.login,
+  }) : size = 16;
+  Avatar.small({
+    @required this.url,
+    this.login,
+  }) : size = 24;
+  Avatar.medium({
+    @required this.url,
+    this.login,
+  }) : size = 36;
+  Avatar.large({
+    @required this.url,
+    this.login,
+  }) : size = 48;
 
   @override
   Widget build(BuildContext context) {
@@ -21,8 +37,8 @@ class Avatar extends StatelessWidget {
       child: FadeInImage.assetNetwork(
         placeholder: 'images/octoface.png',
         image: url,
-        width: 2 * size,
+        width: size,
-        height: 2 * size,
+        height: size,
         fadeInDuration: Duration(milliseconds: 200),
         fadeOutDuration: Duration(milliseconds: 100),
       ),

lib/widgets/comment_item.dart

@@ -43,9 +43,8 @@ class CommentItem extends StatelessWidget {
       crossAxisAlignment: CrossAxisAlignment.start,
       children: <Widget>[
         Row(children: <Widget>[
-          Avatar(
+          Avatar.medium(
             url: payload['author']['avatarUrl'],
-            size: 16,
             login: payload['author']['login'],
           ),
           SizedBox(width: 8),

lib/widgets/event_item.dart

@@ -79,10 +79,9 @@ class EventItem extends StatelessWidget {
             Row(
               crossAxisAlignment: CrossAxisAlignment.start,
               children: <Widget>[
-                Avatar(
+                Avatar.medium(
                   url: event.actorAvatarUrl,
                   login: event.actorLogin,
-                  size: 20,
                 ),
                 SizedBox(width: 10),
                 Expanded(

lib/widgets/issue_item.dart

@@ -120,10 +120,9 @@ class IssueItem extends StatelessWidget {
                             children: <Widget>[
                               // FIXME: Deleted user
                               if (payload['author'] != null) ...[
-                                Avatar(
+                                Avatar.extraSmall(
                                   login: payload['author']['login'],
                                   url: payload['author']['avatarUrl'],
-                                  size: 8,
                                 ),
                                 SizedBox(width: 4),
                                 Text(

lib/widgets/repository_item.dart

@@ -130,7 +130,10 @@ class RepositoryItem extends StatelessWidget {
         child: Row(
           crossAxisAlignment: CrossAxisAlignment.start,
           children: <Widget>[
-            Avatar(url: payload['owner']['avatarUrl'], size: 12),
+            Avatar.small(
+              url: payload['owner']['avatarUrl'],
+              login: payload['owner']['login'],
+            ),
             SizedBox(width: 8),
             Expanded(
               child: Column(

lib/widgets/user_item.dart

@@ -50,7 +50,7 @@ class UserItem extends StatelessWidget {
         child: Row(
           crossAxisAlignment: CrossAxisAlignment.start,
           children: <Widget>[
-            Avatar(url: avatarUrl, size: 24),
+            Avatar.large(url: avatarUrl),
             SizedBox(width: 10),
             Expanded(
               child: Column(