In Aseprite, "Brush Size" behaves differently depending on which you have selected. You can access the Brush Types by right-clicking anywhere on the canvas or via the context toolbar.
Located at the top of the interface just below the main menu bar. aseprite brush size
Use Pixel Perfect mode for outlining characters with a 2px or 3px brush size. It automatically cleans up messy corners. In Aseprite, "Brush Size" behaves differently depending on
Ever wonder exactly how big your 12px brush is compared to your sprite? aseprite brush size
If you have a graphic tablet or prefer a visual approach:
You don't need to switch tools to erase. You can temporarily turn your current brush into an eraser based on its size.