Every time I'm trying to paint something, the program picks its own brush size which is way too big.
Is this a bug or a feature?
Nope, most likely you incorrectly captured the paint controls area. To reset this, go to Options, and click Forget Coordinates. Now you will be able to capture the paint controls area coordinates again. Refer to the in-app tutorial or the demo video on youtube for a visual aid. Let me know if you are still having trouble!