I just made my first game(a maze which I think everyone does on their first attempt), and now I compiled it and it turned out to be a huge 900mb+ file. I realized that most of this came from the starter content, and as I dont have access to better stuff, I have to stick to starter content for now :(. So, I learnt that I can’t import .uasset files. If I want to say use the floor, I want to import it and then use it to make walls(thus no need to add them as well). Also, I dont think I will need all the different particles etc. as well.
I first thought that I could add in the starter content and then delete everything I don’t need but as I started this process, I could not decide if I need the particle systems or not, which of the textures should I keep etc. THe main reason for this is that I want to start with something basic, making changes as I see the videos until I have a good grasp on UE4. So, is there a way to add single .uasset files one by one when necessary into an existing project?
Also, how does a person make UE4 .uasset files. All my searches turned out to form .fbx files and the like.
If you want to transfer files between projects just select the mesh, particle, blueprint, or whatever you want to transfer in the Content Browser. Right click and use the Migrate function then it will send it and all the dependent files (such as the materials & textures if you’re sending a mesh) into the other target project.
You could just create a separate project with the starter content -when you need something just open that project - right click onto the mesh/sound/particle/… - migrate - choose your game project 
.uasset files are created by the engine when you import/create assets -> when you import a fbx file, it will automatically saved as a .uasset file
Edit:
@: Yep, thats right ^^
Thanks a lot both of you