collectgarbage("count")which gives you some idea of the overall memory usage. You can combine this with the
EVENT_ADD_ON_LOADED (integer eventCode,string addonName)event however the data you get doesn't seem to make any sense based on either the size of the addon on disk or how much memory it actually uses. This event is called for each addon loaded but the memory usage between calls is all over the place and doesn't seem to related to actual memory usage of the addon.