Sprites are pixelated after resizing

Hello. I'm pretty new to Unity and my English is not great, so I apologize for that. So I created some sprites for a 2D game. I sized them how they should look at 1920x1080 screen resolution. When I want to resize them for lower resolution, they start to be pixelated around the edges. So my question is, how to create sprites, that can be used in multiple resolutions without loosing quality. I searched on the web, but I couldn't find anything. Thanks for your time.