Edit: this is really easy to use wow.
How do I resize my images to be small like yours? And how did you make a circular shape like that?
On the right side of the screen, there is a resize option. For Codex awards, the image size needs to be 25x25. If you want it to be that small, I recommend starting that small, instead of working with 32x32 and resizing after you've already made the sprite. You lose a lot when you resize versus just starting in the smaller size.
As for making a circle,
just zoom into my marble and look at the number of pixels on each side and use the mirror tool (so it makes both sides identical).
Here is a close up of my marble for you