In terms of file sizes, not really as long as the size is justified, such as data files or images that are integral to the plugin. TerrainMap, for instance, is over 30 MB but most of that is due to the need to have pre-zoomed tile images available. Some users may balk at downloading a plugin that is many MB in size, but realistically, hard drive storage is cheap and a few MB more or less isn't going to be noticeable on any modern system. In terms of impact on the game client, the file sizes will slow down the loading time for the plugin (usually imperceptible since it takes a fraction of a second for most plugins) since every line has to be parsed, so you would not want to include a ton of unnecessary documentation in every file, add readme files that don't get loaded (and thus not parsed) if necessary.
The actual memory allocation at any point in time however is extremely significant as it will add to the overall memory footprint of the game client and will eventually cause stuttering (due to heavy disk access) and may in extreme cases crash the client. So, if you are done with objects, make sure you remove any handles to them so that the automatic garbage collection can destroy the objects and deallocate the memory they occupy.
Actually, the more common culprit in terms of load times is saved data. Plugindata files can get pretty big depending on what is being stored in them. That is one of the reasons that custom annotation data is stored per map in MoorMap and only the data for the currently displayed map is actually loaded and displayed at any time. If the data can be separated into logical chunks that can be loaded as needed (a bit difficult due to Turbine's attempts to limit real-time access to files to prevent botting, but doable), you are generally better off with many smaller files than one huge file, just be sure to give them a nice prefix that makes them easy to identify if a user wants to delete them (for instance, MoorMap data files start with "mm"). Note, there is also a maximum table size for reading data in from plugindata files (not due to a Lua restriction, but rather a glitch in the way that Turbine implemented plugindata file parsing). I don't recall off the top of my head what the maximum size is, but you can create a table in a plugin that is so large that once saved, it won't properly reload. The library files for the current (unpublished) version of IRV are so massive that some of them have to be broken down into multiple parts and loaded in pieces - and yes, IRV is one of those plugins that can bring the client to its knees for several minutes when loading or unloading
which is one of the reasons that I never published the updated version, but it is a nearly indispensable tool for me when looking for specific images (without accessing the dat files directly which violates the EULA).