diagnostic: implement cache size check

Library/Homebrew/diagnostic.rb

@@ -835,6 +835,17 @@ module Homebrew
         EOS
       end
 
+      def check_for_large_cache
+        return unless HOMEBREW_CACHE.exist?
+        return if ENV["CI"] # CI can be expected to have a large cache.
+        cache_size = HOMEBREW_CACHE.disk_usage
+        return unless cache_size > 2_147_483_648
+        <<~EOS
+          Your HOMEBREW_CACHE is using #{disk_usage_readable(cache_size)} of disk space.
+          You may wish to consider running `brew cleanup`.
+        EOS
+      end
+
       def check_for_other_frameworks
         # Other frameworks that are known to cause problems when present
         frameworks_to_check = %w[