It depends on whether you want to modify the game files directly, or if you want to add custom assets through the plugin API. I guess you're referring to the new version of RW? If you want to add models, textures or sounds through our API, I can't give much information at this stage unfortunately, since the plugin API of the new version isn't fully fleshed out yet. In general, I can just give these information:
Sounds: This will work just like in the Java version. This means you can load sound files through the API and play them for a certain player. Supported file formats: ogg, mp2, mp3, midi/mid, wav. The API enables you to dynamically change volume, pitch and a few other parameters when playing a sound. You can also determine whether or not a particular sound file should be streamed (useful when playing large sound files like music tracks)
Textures: You can use any texture sizes you want, but it's recommended to not exceed 2K (due to the file size: in multiplayer, players have to download the plugin assets from the server, and if you add too many large files, it will take a long time for players with slow internet connections to join the server).
In general, it's always a good idea to keep the texture size as small as possible. Keep the memory consumption (especially the VRAM consumption) on the client in mind
Models: Can't say much about them yet, right now only .obj files are supported by the API, but we're looking for support of other formats as well.
Workbenches in the new version, for example, have a triangle count of ~ 5000. 1 unit is equal to 1 block (i.e. approx. 0.5 meters)
Animations: Can't say much about that either. Currently the API has no support for animations (except some basic transform commands you can issue through the API, e.g. move an object to a certain location etc). We first have to support a model format which is capable of including animation data (e.g. .fbx).
Alternatively we have been looking into adding support for prefabs created within Unity - this way you could either load or create new animations in Unity and put them into an asset bundle (which could then be loaded by the API). I can't say for sure if this will be added eventually, but it's on our "take-into-consideration" list 
Thanks for such a quick and high quality reply and thanks for the specs and recommendations while RW2 is under construction.
Near term I'm making models while gaining proficiency with Unity and Blender and eventually learning the API as RW2 gets more complete.
I was planning on making mods using the API to sell BUT after reading forum comments I agree they should be free.
I'd love to work with others to make mods and things for RW2 and have fun testing.
Can’t wait until you are able to make mods Elvaeus!
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