Universal RP renders all real-time shadows for a frame using one common shadow map atlas for all punctual light shadows (i.e shadows for Spot Lights and Point Lights), and an other shadow map atlas for Directional Light shadows.
Set the size of these atlases in your Unity Project’s Universal Render Pipeline Asset. The atlas size determines the maximum resolution of shadows in your sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
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For example, an atlas of size 1024 x 1024 can fit:
If you want to make sure that the resolution Universal RP uses for a specific punctual light shadow in your project, will not go under a specific value: Consider the number of shadow maps required in the scene, and select a big enough shadow atlas resolution.
For example: if your scene has four Spot Lights and one Point light ; and you want each shadow map resolution to be at least 256x256. Your scene needs to render ten shadow maps (one for each Spot Light, and six for the Point Light), each with resolution 256x256. Using a shadow atlas of size 512x512 would not be enough, because it can contain only four maps of size 256x256. Therefore, you should use a shadow atlas of size 1024x1024, that can contain up to sixteen maps of size 256x256.