I’m trying to create a 2D (entity?), not sure if that’s the right word, that grows each time it comes in contact with another object/bullet. I don’t really know how to start.
If you think of your goals as pieces, that might help you begin.
- Get 2d object/entity (know how to get one? put it in the scene?)
- detect collision
- change size/scale (upon hit)
I have a feeling if you look around for those “pieces” of your puzzle, you will find that there is a lot out there in the world for you. If you try some things you find/learn, and it’s still not working correctly, you can always return to the forums and ask a question (and show your code!) 
Consider trying some Unity tutorials (on the website) to help get you familiar with how a project comes together, as well as with learning some useful aspects of the engine, etc…