Most, if not all your data is usually found in your User folder.
- Computer
- Double-click C:
- Double-click Users
- Right-click on your User account, choose Properties
You will then see an estimate.
If you have other users on your system, you will need to repeat the procedure to get the size of their storage. Add them together.
USB thumb drives are not a great place to put a backup. They are much less reliable and slower than using an external USB connected hard drive. After you have an estimate of the space you need, you can buy yourself the appropriate size. Best advice is to buy much bigger than you currently need because your need is very likely to grow.