You can make whatever you want from it, the limiting factor your imagination, your knitting skills and the size of your T-shirt. You can stitch multiple strings of yarn together to make a bigger ball.
Let's show you the process to do this.
Step 1: Type of T-shirt to Use and How to Cut It.
Ideally a top as you will have less waste, or if you have a regular T-shirt you will need to cut it just where the sleeves start, so only the bottom part is used. So you'll end up with a piece of cloth with no joints or seams.
Depending on the effort you want to put into it, you can either cut of the bottom seam or open it up
Step 2: Cutting the Strips Into Yarn
The width of the strip will determine the thickness of your yarn, but the minimum is 15 mm, anything less than this, the process will fail.
When done cutting unfold it.
Step 3: Cut a Continuous Strip & Roll Your Yarn Into Ball
Cut the strips at an angle from one strip to the next creating one continuous strip.