Is there any way in Unity 5 editor to see which Game Objects and Prefabs i am not using inside any scene? So i can delete all and make game smaller size.
There is no way to check it easily (although codes can be written and there are a few plugins for this).
On the other hand, you can check if something is “referenced/used” in a scene, by right clicking on the prefab in the project view and select “find references in scene”.
Game size won’t be effected by prefabs and assets if they are not in a “Resources” folder.
Also, if you want to see how much is the built version affected by prefabs/assets you can check out the build log after you’ve built it (top right corner in the console window and “open editor log”, look around the end of the file and you will find it with lots of details).