Fixes #1216

css/converse.css

@@ -9768,6 +9768,7 @@ body.reset {
     -webkit-animation: spin 2s infinite, linear;
     -moz-animation: spin 2s infinite, linear;
     animation: spin 2s infinite, linear;
+    width: 1em;
     display: block;
     text-align: center;
     margin: 2em;

sass/_core.scss

@@ -402,6 +402,7 @@ body.reset {
 
     .spinner {
         @include animation(spin 2s infinite, linear);
+        width: 1em;
         display: block;
         text-align: center;
         margin: 2em;