The size values are in plugins \ Default \ Monsters \ StandardMonsterPlugin.cs, the radius values for the MapShapeDecorator of EliteChampionDecorator, EliteMinionDecorator, and EliteLeaderDecorator. To edit the values without editing any files from the Default folder, you could add the desired config values into an existing customization plugin, like plugins \ User \ PluginEnablerOrDisablerPlugin.cs
Code:
//change the default sizes of the elite circles on the minimap
Hud.RunOnPlugin<StandardMonsterPlugin>(plugin => {
((MapShapeDecorator)(plugin.EliteLeaderDecorator.Decorators[0])).Radius = 10;
((MapShapeDecorator)(plugin.EliteChampionDecorator.Decorators[0])).Radius = 10;
((MapShapeDecorator)(plugin.EliteMinionDecorator.Decorators[0])).Radius = 8;
});