Use a getterSetter to manage int values in settings.

services/web/app/views/project/editor/left-menu.pug

@@ -136,7 +136,8 @@ aside#left-menu.full-size(
 				label(for="fontSize") #{translate("font_size")}
 				select(
 					name="fontSize"
-					ng-model="settings.fontSize"
+					ng-model="fontSizeAsStr"
+					ng-model-options="{ getterSetter: true }"
 				)
 					each size in ['10','11','12','13','14','16','20','24']
 						option(value=size) #{size}px

services/web/public/coffee/ide/settings/controllers/SettingsController.coffee

@@ -8,6 +8,11 @@ define [
 		if $scope.settings.pdfViewer not in ["pdfjs", "native"]
 			$scope.settings.pdfViewer = "pdfjs"
 
+		$scope.fontSizeAsStr = (newVal) ->
+			if newVal?
+				$scope.settings.fontSize = newVal
+			return $scope.settings.fontSize.toString()
+
 		$scope.$watch "settings.theme", (theme, oldTheme) =>
 			if theme != oldTheme
 				settings.saveSettings({theme: theme})