Is this a bug or something? It’s been happening to me since last year I thought recompiling aseprite again with the newest version will work but it didn’t…
Can some help me? 
I’m using a 1.3.9-dev (compiled) (windows) version of aseprite.
i think you have stylus support on, so unless you are using a drawing tablet, it will remain a single pixel as that is the smallest size, you can turn it off in the icon that is right next to the ink icon